Outlining the Mechanism of Flame Retardancy in Polyamide 66

Fire Safety Journal 70 (2014) 46–60

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Fire Safety Journal

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Outlining the mechanism of ﬂame retardancy in polyamide 66 blended with melamine-poly(zinc phosphate)

Anil D. Naik a, Gaëlle Fontaine a, Fabienne Samyn a, Xavier Delva b, Jérémie Louisy b, Séverine Bellayer a, Yann Bourgeois b, Serge Bourbigot a,n a ISP/UMET—UMR/CNRS 8207, Ecole Nationale Supérieure de Chimie de Lille (ENSCL), Avenue Dimitri Mendeleïev—Bât. C7a, BP 90108, 59652 Villeneuve d’Ascq Cedex, France b Floridienne Chimie, 12 Quai des Usines, 7800 Ath, Belgium article info abstract

Article history: Glass-fiber reinforced polyamide 66 is flame retarded with a mixture of melamine-poly(zinc phosphate), Received 16 May 2014 (Safires400) and diethyl aluminium phosphinate. The performance of this synergistic combination of Received in revised form additives is multi-modal and a comprehensive investigation is undertaken to elucidate the underlying 27 August 2014 flame retardancy mechanism. The strategy was to characterize the different chemical species responsible Accepted 31 August 2014 for flame retardancy that are generated in gas and condensed phases under different fire scenarios. Available online 10 October 2014 Following heat release rate (HRR) curve of flame retarded polyamide formulations obtained by mass loss Keywords: calorimeter, samples in different stages of degradation are collected and investigated. Further flame Flame retardancy retardants and formulations were degraded in tubular furnace whose temperature protocol relied on Polyamide-66 thermal degradation profile obtained from thermogravimetric analysis (TGA). In either case, species Melamine polyphosphate generated in condensed phase were studied by solid state nuclear magnetic resonance spectroscopy Aluminum phosphinate 27 31 13 Zinc polyphosphate (magic angle spinning (MAS) NMR; Al, P and C), Fourier transform Infra-red spectroscopy (FTIR), Safires400 X-ray powder diffraction (XRD), electron probe microanalysis (EPMA), scanning electron microscopy (SEM), and optical microscopy, whereas TGA coupled FTIR, and pyrolysis gas chromatography mass spectrometry (Py/GC/MS) were utilised to investigate species released in gas phase. Flame retardancy mechanism is elaborated based on the identification of the chemical species in both gas and condensed phases and their specific contributing role. & 2014 Elsevier Ltd. All rights reserved.

1. Introduction recycling-waste disposal, and theoretical studies involving simulation of fire scenario. Fire risks involving polymers have devastating consequences as Among several inorganic flame retardants (FRs), metal phos- polymer utility ranges from consumer products to high-performance phates have been an efficient ingredient in the formulation of flame hardware manufacturing [1–6].Apanelofflame retardants are retardant polymers particularly with cations like Zn2þ ,Mg2þ ,Al3þ, developed to confront these issues and safeguard vulnerable targets Zr4þ ,Ti4þ [3,4,6–12].Midstofthem,zincphosphates(ZnPO)are like E&E products, materials used in transport vehicles, building- found to be multi-faceted in flame retardancy [12]. ZnPOs have rich construction materials, upholstery etc … depending on the type of structural chemistry mainly due to the coordination flexibility of the plastics and targeted application. The current preventive fire safety zinc ions, which can form polyhedral by coordinating to four, five or trend, aimed at keeping the highest fire safety standards, explores six donor atoms in the framework. Such zinc polyhedra have the novel additives to insulate polymers from fire hazards that are not ability to catenate to chains by face, edge, or vertice-sharing [13]. only efficient, synergistic, multifunctional, but are tailored to be tagged ZnPO also exhibits a wide glass forming composition ranging from with ‘eco-label’ to comply with stringent regulations [3,7].Further, ultraphosphate region (y¼ZnO/P2O5o1) to highly cross-linked poly- flame retardancy mechanism is multifaceted and an insight into its phosphate glasses (yZ1) [14–18]. The basic structural unit of all mode of action provide valuable information for issues like additive- phosphates, crystalline or amorphous, is the PO4 tetrahedron. polymer compatibility, synergism, re-design strategy for additives, Coupling such efficient metal phosphates with tailored organic molecules is promising strategy to further upgrade phosphates to attain multi-functionality. In this respect, flame retardancy label of melamine family was further widened and strengthened by introdu- n Corresponding author. cing melamine intercalated metal polyphosphates [8,9,11,19,20]. s E-mail address: [email protected] (S. Bourbigot). Safire series is a good example for this that was introduced recently http://dx.doi.org/10.1016/j.firesaf.2014.08.019 0379-7112/& 2014 Elsevier Ltd. All rights reserved. A.D. Naik et al. / Fire Safety Journal 70 (2014) 46–60 47 by Wehner and Dave [8,9].Safires400 (hereafter referred S400), plastics (St Fons, France), Diethyl aluminium phosphinate with Melamine‐poly(zinc phosphate) is one among them. The molecular trade name of Exolit OP1230 (AlPi) was from Clariant (Knapsack, s structure of S400 is given in Fig. 1a. This molecule can also be Germany). Safire 400, Melamine‐poly(zinc phosphate), {(MelH)2 considered as a categorized molecular state of melamine which [Zn(P2O7)]}n) was kindly provided by Catena Additives a fully shares the responsibility of charge balancing, structure directing owned subsidiary of Floridienne Group (Ath, Belgium) and are moiety besides actively participating in flame retardancy. On a supplied in polycrystalline form (Fig. 1b and c). molecular level, the cationic chains of melamine are assumed to be alternated by anionic ZnPO chains with dense secondary interactions 2.2. Formulations, processing and sampling with the matrix similar to melamine phosphates [21]. fi fl fi S400 is found to have signi cant in uence on re performance of 2.2.1. Formulation and processing fi glass- bre reinforced polyamide 66, particularly when coupled with Compounding of formulations (Table 1) were performed using diethyl aluminium phosphinate and passes mandatory tests like UL- HAAKE Rheomix OS PTW 16 twin screw extruder [11a]. All the 94, and glow wire tests. Further there is substantial reduction in peak materials were dried at 80 1C under vacuum for 24 h prior to of heat release and total heat release evaluated by mass loss extrusion. calorimeter [8,9,11]. S400 is also found to be a char/intumescence forming agent. Our investigation has been focussed on the elucida- 2.2.2. Sampling tion of flame retardancy mechanism of S400 and aluminium Samples for the study were collected from two sources (Table 1). phosphinate (1:2 ratio, total 18 wt% of flame retardants) blended One is from mass loss calorimeter experiment which is based on HRR glass fibre reinforced polyamide-66. There are several comprehensive curve [11a] wherein exposure to heat flux/flaming were stopped reports on mechanistic aspects of melamine based flame retardants and extinguished manually at 100, 300, 600 and 1000 s. The intended in polyamides [22–33]. Our study has been supported by parallel (100 to 1000 s) time selection for sample collection is principally based investigation on decomposition of neat S400, and single additive on HRR profile of PA66/GFþAlPiþS400 formulation which approxi- (S400) blended glass fibre reinforced polyamide-66. Further supple- mately denotes ascending, peak, descending and end of degradation, mentary data on neat polymer, aluminium phosphinate, and alumi- respectively. The same profile is extended for single additive formula- nium phosphinate blended polymer are also recorded and used as tions and neat PA66/GF. Most of the results investigated are based on references. Samples at different stages of degradation were collected these tests. Pyrolysis experiments were performed in tubular furnace based on heat release rate (HRR) curve from mass loss calorimeter. under nitrogen atmosphere. At the selected temperature, the sa- Samples were also pyrolysed in tubular furnace based on thermal mple placed in ceramic crucible was introduced into the quartz tube decomposition pattern obtained from thermogravimetry. The poten- fixed inside the tubular furnace and kept for 3 h. The tempera- tial chemical species formed during the decomposition pathway of ture selection is derived from TGA profile of formulations/additives these formulations in either method were tracked with solid state which corresponds to temperature of maximum weight loss in each nuclear magnetic resonance spectroscopy (MAS NMR; 27Al, 31Pand step performed at heating rate of 20 1C/min under nitrogen atmo- 13C), Fourier transform Infra-red spectroscopy (FTIR), and X-ray sphere. Additional temperatures mentioned in Table 1 under this powder diffraction (XRD). The gaseous products released during section correspond to either a stable plateau at the end of major the thermal decomposition of these formulations/additives were degradation step or completion of decomposition. To have an characterised by thermogravimetric analysis (TGA) coupled FTIR and pyrolysis gas chromatography mass spectrometry (py/GC/MS). Morphology of intumescence/char and additives were studied by Table 1 optical microscopy, electron probe microanalysis (EPMA), and scan- Materials investigated in the present study and conditions of sample collection. ning electron microscopy (SEM). The results are gathered to establish Materialsa Sample collection timeb T (1C) profilec themodeofactionofflame retardants in the flame retardancy of (in second) polyamide 66. PA66/GFþAlPiþS400 100, 300, 600, 1000 421, 535, 750 PA66/GFþS400 100, 300, 600, 1000 374, 468, 578, 750 PA66/GFþAlPi 100, 300, 600, 1000 414, 542 2. Experimental PA66/GF (neat) 100, 300, 600 401 S400 (neat) – 376, 554, 750 AlPi (neat) – 452, 510 2.1. Materials AlPiþS400 (2:1 ratio, – 421, 535, 750 manually mixed) Abbreviations for the materials used are given in bracket and a AlPi: 12 wt%; S400, 6 wt%. are followed throughout this work. Polyamide 66 reinforced with b Based on HRR curve. 30% glass fibers (PA66/GF) was supplied by Rhodia engineering c For tubular furnace experiment.

Fig. 1. (a) Molecular structure of S400. ((b) and (c)) SEM images on powder sample of S400 showing polycrystalline morphology. 48 A.D. Naik et al. / Fire Safety Journal 70 (2014) 46–60 insight into the potential mutual interactions between additives, elemental analysis. The char surface was transferred on a conductive additive mixture (AlPiþS400 in 2:1 ratio as in formulation) were carbon tape and carbon coated with a Bal-Tec SCD005 sputter coater. also pyrolysed in tubular furnace and the decomposed samples are Elemental mapping (N, Zn, Si, P, Al, Ca) was performed over an area studied. Degraded samples studied in the present work were surrounding a glass fiber. The analyses were carried out at 15 kV, 15 nA labelled with a name followed by a number. This number indicates for back scattered electrons (BSE) images and at 15 kV, 40 nA for either the time at which the mass loss calorimeter experiment is silicon (Si), calcium (Ca), phosphorus (P), aluminum (Al) and zinc (Zn) stopped (100, 300, 600, or 1000 s) or temperature of maximum X-ray mappings. For mappings, a TAP crystal was used to detect the Si degradation in each step of TGA curves. Results of tubular furnace and Al Kα X-ray, and a PET crystal to detect the Ca and P Kα X-ray. experiments are supplementary and discussed only wherever necessary in the paper. 2.3.5. TGA-FTIR analysis 2.3. Instrumental Gas phase analysis were carried out in TGA Q5000 (TA instruments) coupled with FTIR Nicolet iS10 spectrometer (Thermo- 2.3.1. Thermal analysis Fischer). Samples (15 mg) were heated in a 100 μL alumina crucible Thermogravimetric analyses (TGA) were performed using SDT from 50 1Cto8001C with a heating rate of 10 1C/min under nitrogen – Q600 (TA instruments). Samples (approx. 6 8mg)wereplacedin atmosphere. A balance purge flow of 15 mL/min and a sample purge open alumina pans covered with gold foil to avoid possible reactions flow of 100 mL/min was maintained. A transfer line with an inner between pan and phosphorus contents of the samples and heated diameter of 1 mm was used to connect TGA and infrared cell. The 1 under nitrogen atmosphere with a heating rate of 20 C/min. temperature of transfer line and gas cell was kept at 225 1C. Prior to this, samples were maintained for 2 h under nitrogen stream. IR 2.3.2. Mass loss calorimeter spectra were collected in 400–4000 cm1 spectral range with a total The mass loss calorimeter (Fire Testing Technology (FTT)) is of 450 scans. used for collecting sample for analysis. Plates (100 100 3mm3 plates) for calorimeter test were made via compression molding using DARRAGON press apparatus. Plates were wrapped in alumi- 2.3.6. Pyrolysis GC/MS nium foil leaving the upper surface exposed to the heater and Samples (200 μg) were analyzed by Pyrolysis GC–MS (Shi- placed in horizontal position on ceramic block encased in a madzu, GCMS-QP2010 SE). GC separation was carried out with a metallic container at a distance of 40 mm from cone base. External fused silica capillary column (SLB 5 ms) of 30 m length and heat flux of 50 kW/m2 was used for all the experiments. 0.25 μm thickness. Analyses were carried out in direct pyrolysis mode. The temperature selection is based TGA pattern of con- 2.3.3. Spectral and XRD analysis cerned sample. The furnace is set for the final temperature and FT-IR spectra were recorded on Nicolet Impact 400 D spectro- sample is pyrolysed for 0.5 min. Helium was used as a carrier gas fl meter in Attenuated Total Re ection (ATR) mode in the range at pressure of 120 kPa with a split ratio of 50. The transfer line was – 1 500 4000 cm at room temperature. To minimize the signal to maintained at 275 1C. Column temperature is programmed in the noise ratio, the spectra are acquired as a result of 32 scans with a following way. The initial column temperature was held at 35 1C 1 resolution of 4 cm . for 1 min followed by a temperature ramp at 10 1C/min to a final X-ray powder diffraction (XRD) spectra were recorded using a temperature of 300 1C and isotherm for 20 min. The MS was 1– 1 λ ¼ Bruker AXS D8 diffractometer in the 5 60 range at RT. ( Cu Kα operated under Electron Ionization EI mode. An online computer fi 1.5418 Å, 40 keV, 25 mA) in con guration 2-theta/theta. The acq- using GCMS real time analysis and PY-2020i software controlled 1 uisition parameters were as follows: a step of 0.02 ,asteptimeof2s. GC/MS system. The eluted components were identified by library 31 P NMR measurements have been performed on a Bruker search and only significant peaks observed in the total ion Avance II 400 at 40.5 MHz using a 3.2 mm probe, with/without chromatograms were studied and compared to a mass spectral 1 –31 cross polarization (CP, H P), with dipolar decoupling (DD) and database (GCMS postrun analysis, and NIST). magic angle spinning (MAS) at a spinning speed of 20 kHz. The delay time between two pulses was fixed at 120 s without CP and 5 s with CP. The spectra were acquired with 32 scans. The reference used was 85% H3PO4 in aqueous solution. 27Al NMR measurements have been performed on a Bruker Avance II 400 at 104 MHz using a 3.2 mm probe, with MAS of 20 kHz. The delay time between two pulses was fixed at 1 s. The spectra were acquired with 1024 scans. The reference used is 1 M solution of aluminum nitrate. 13C NMR measurements have been performed on a Bruker Avance II 400 at 100.4 MHz using 3.2 mm probes, with CP 1H–13C, dipolar decoupling (DD) and MAS of 10 kHz. For all samples, a delay time between two impulsions of 5 s and a contact time of 1 ms were used. The spectra were acquired with 1024 scans. TMS is used as reference for the chemical shift.

2.3.4. Microscopy Morphology of samples was studied using scanning electron microscope (SEM), (Field emission gun (FEG) Hitachi S4700). VHX digital optical microscope (Keyence, VH-Z 100R) was used to investigate texture of samples. An electron probe microanalyser (EPMA) Fig. 2. TG curves for formulations and additives (20 1C/min heating rate, under using wavelength dispersive X-ray spectrometers was used to perform nitrogen atmosphere). A.D. Naik et al. / Fire Safety Journal 70 (2014) 46–60 49

3. Results measured based on HRR curve (Fig. 3a) with associated parameters [11]. As seen from HRR curve, neat PA66/GF burns easily with a 3.1. Thermal decomposition behaviour and mass loss calorimetry high peak of heat release rate (pHRR) value of 354 kW/m2 and results totally decomposed leaving behind only glass fiber residue. Addi- tion of single component FR, like AlPi (pHRR, 149 kW/m2)orS400 Thermal decomposition profile of additives and formulations (106 kW/m2) alone in PA66/GF reduces the pHRR by 58 and 70% are displayed in Fig. 2 and thermal parameters are summarized in respectively and results in small amount of char (Fig. 3b). The Table 2. S400 additive exhibits two unequal decomposition steps optical microscope image on the residue of PA66/GFþS400 shows yielding to significant amount of residue. The second step is very that surface is not continuous but full of voids and is mainly formed gradual and weight loss continues until 770 1C. The FRs modify the with a mesh of glass fibre sparsely holding char (Fig. 3dande).The initial thermal stability of PA66/GF and its thermal degradation synergy between the two additives is clearly evidenced in HRR pattern. Single additive formulation PA66/GFþS400 undergo decom- curve of PA66/GFþAlPiþS400 with significant reduction in pHRR position in three steps but a short plateau is found only between 500 (62 kW/m2; 82% reduction). The value of total heat release (THR) and 580 1C. PA66/GFþAlPiþS400 decompose in a single major step for PA66/GF is 77 MJ/m2 and is reduced to 43 MJ/m2 in PA66/ also exhibiting a plateau in the same region with a significant residue GFþAlPi and nearly same for PA66/GFþS400 (36 MJ/m2) and two weight. A stable species is formed in this temperature range which additive formulation (31 MJ/m2). Another important parameter is gradually decomposes compared to PA66/GFþS400 (Table 2). While time to ignition (TTI) which is 72 s for PA66/GF, slightly less for neat PA66/GF decomposes completely, its two additive counterpart PA66/GFþAlPi (43 s) and PA66/GFþS400 (66 s). On the other hand, yields significant amount of char. PA66/GFþAlPiþS400 has been efficient in resisting the heat flux by The response and performance of formulations to a simulated prolonging TTI to 111 s. It is to be noted that degradation is fire scenario is evaluated by mass loss calorimeter and they are extended over a period of time with a secondary HRR peak and there is contribution to char/intumescence. During the mass loss calorimeter experiment it is observed that Table 2 fl Thermal parameters for additives and formulations (20 1C/min heating rate, under under the high heat ux, the upper surface of polymer plate nitrogen atmosphere). undergoes bubbling until it starts to swell. The bubbles shrink into irregular but continuous coarse structure that forms a glassy Compound Steps T (1C) range Weight Residue intumescence (Fig. 3c). This shiny envelope is pushed upwards (major) loss (%) weight (%) by the pressure of released gases produced by the burning PA66/ 1 309–502 55.1 34.6 materials underneath forming a dome shaped intumescent struc- 30GFþAlPiþS400a ture. This is crack-free, hollow and supported with densely inter- PA66/GFþS400 3 278–430 37.0 32.2 woven glass-fibres (Fig. 3g) making a strong protective barrier – 430 518 19.5 between un-degraded polymer and heat flux. Optical microscope 518–699 09.4 PA66/GFþAlPi1 1341–504 53.1 35.7 study on intumescence surface reveals presence of numerous S400 2 325–402 14.2 44.4 protrusions resembling ‘cells’ (Fig. 3f). 402–780 39.5 The flammable and non-flammable gases released before and AlPi 2 420–466 40.2 42.3 after intumescence play a key role in controlling the parameters of 474–541 17.0 the HRR curve. Types of volatiles can be conveniently traced PA66/GF 1 337–504 68.8 31.2 and their evolution is mapped by in situ TGA-FTIR and further a There is gradual weight loss until 800 1C after the first stage of decomposition. confirmed by pyrolysis GC–MS. These are described in the

Fig. 3. (a) Mass loss calorimeter response of formulations. (b) Residue in PA66/GFþS400 formulation. (c) Intumescence in PA66/GFþAlPiþS400 formulation. ((d) and (e)) Optical microscopic images on PA66/GFþS400 residue. (f) Intumescence surface in PA66/GFþAlPiþS400. (g) Undersurface of intumescence in PA66/GFþAlPiþS400. 50 A.D. Naik et al. / Fire Safety Journal 70 (2014) 46–60

Fig. 4. TGA-FTIR under nitrogen atmosphere of (a) PA66/GF (b) PA66/GFþAlPiþS400. following sections. Chemical species responsible for ﬂame retardancy embedded in the intumescence/char are discussed under condensed phase analysis.

3.2. Gas phase analysis

3.2.1. FTIR-TGA studies The evolved gas analysis carried out for PA66/GFþAlPiþS400 is displayed in Fig. 4 andcomparedwithPA66/GF.Identification of gases in the FTIR spectra is based on earlier reports and relating to characteristic bands [26,29,31,34–36]. Neat PA66/GF shows no significant evolution of volatiles until 30 min (300 1C). There- after simultaneous but gradual evolution of hydrocarbon (3000– 1 1 2800 cm ), CO2 (2368, 667 cm ), cyclopentanone or its derivative (C¼O, 1767 cm1), and ammonia (963, 933 cm1), is observed. At 35 min (350 1C) the evolution of cyclopentanone increases drasti- cally along with other above mentioned gases. Around 40 min (403 1C) cyclopentanone starts to decrease and hydrocarbon starts to increase along with ammonia. After 43 min (436 1C) there is only hydrocarbon evolution. The profile in case of PA66/GFþAlPiþS400 deviates compared to neat PA66/GF. Progressive weight loss starts from 21 min (244 1C). Fig. 5. Selected pyrolysis GC/MS chromatograms (a) for PA66/GF, pyrolysis at 1 þ þ 1 1 Around 29 min (322 1C) slightly earlier than in PA66/GF, evolution 420 C; For PA66/GF AlPi S400, (b) pyrolysis at 290 C, (c) pyrolysis at 350 C, (d) pyrolysis at 420 1C. Chemical species (a)–(c) are defined in Table 3. of CO2 begins. At about 34 min (370 1C) which is the inflection point of major degradation step, hydrocarbon (2972 cm1), cyclo- 1 pentanone (1761 cm ) and phosphinic acid (3651, 1277, 1240, of ammonia, hydrocarbon and CO2. Weak signals for AlPi (1162, 851 cm1) starts to evolve. The origin of phosphinic acid is due to 1082 cm 1) are observed at this stage. As the temperature increases, partial degradation of AlPi additive. Ammonia evolution is seen at AlPi is increasingly detected. Thereafter (40 min, 434 1C) CO2 35 min (383 1C). Around 38 min (414 1C), it is mainly evolution decreases considerably but hydrocarbon, ammonia, AlPi dominates A.D. Naik et al. / Fire Safety Journal 70 (2014) 46–60 51 and continues to evolve until 46 min (487 1C). A qualitative Around 350 1C which is the end of second minor decomposition comparison of band intensity of cyclopentanone between PA66/GF step, shows presence of CO2, melamine and small quantity of and PA66/GFþAlPiþS400 indicates a decline in its production in the cyclopentanone. In the next pyrolysis step at 420 1C(Fig. 5c) which latter. Evidence of melamine sublimation and subsequent detection is just after the major weight loss in the third step of decomposi- is not evidenced in the formulation. Either this probability is less or tion in TGA, shows mainly 1,6-hexanediamine. Direct pyrolysis at possibility of its condensation in the transfer line of TGA-FTIR cannot 500 1C on this formulation shows more degraded products of 1,6- be ruled out. In any case ammonia is unambiguously detected, hexanediamine (Table 3). Although identification of phosphorus which is assumed to be evolved after de-ammonation and subse- based species is uncertain due to several unresolved signals, quent condensation of melamine ring which was also confirmed in pyrolysis of neat AlPi at 500 1C shows presence of certain species the degradation of neat Safires200 [11b]. like triethylphosphine oxide (m/z, 134) and diethylphosphine (m/z, 90) possibly rearranged products from degrading diethyl aluminium phosphinate. 3.2.2. Pyrolysis GC–MS studies Fig. 5 depicts selected chromatograms showing release of key 3.3. Condensed phase analysis volatile species in case of PA66/GFþAlPiþS400 and neat PA66/GF under pyrolysis conditions and are grouped in Table 3. Neat PA66/ 3.3.1. FT-IR spectral results GF (Fig. 5a), under pyrolysis at 420 1C (corresponds to maximum 3.3.1.1. Pyrolysis of S400. TheroomtemperaturespectrumofS400 weight stage in TGA) shows presence of water (m/z, 18), CO2 (m/z, (Fig. 6) shows distinct bands corresponding to –NH and –NH2 44), cyclopentanone (m/z, 64), 1,6-hexanediamine (m/z, 116) and stretching between 3090 and 3400 cm 1.Astrongbroadband 1 cyclic monomers of PA66 (Table 3, compound c) predominantly. around 1670 cm is due to v(C¼N) and δ(NH2). The characteristic Degraded products of 1,6-hexandiamine, and aromatic heterocyc- triazine ring band of melamine appeared at 780 cm 1 [39–42]. lic compound (a and b respectively in Table 3) are also detected. Additional bands are identified between 1250 and 950 cm 1,and Pyrolysis at 500 1C (completion of degradation, not shown) shows around 540 cm 1 corresponding to various vibrations from ZnPO comparatively less cyclopentanone [32,37,38] due to further polyhedra from different sources like -meta, -pyro, -ortho phosphate decomposition of it into CO2. Secondary degradation/rearranged and most of the bands are overlapped by melamine vibrations products of 1,6-hexanediamine like 5-hexaneamine, adiponitrile [43,44]. (hexane dinitrile, m/z¼108) and hydrocarbon fragments are also Pyrolysis of S400 in tubular furnace (376 1C, 554 1C and 750 1C) identified. Pyrolysis of PA66/GFþAlPiþS400, is studied at 290 1C, leads to progressive changes in both melamine component and in 350 1C, and 420 1C successively. The first two temperatures corre- ZnPO polyhedra. Melamine component undergoes de-ammonation sponds to minor degradation steps in TGA. Until 290 1C, water, followed by condensation forming more conjugated species or simply ammonia, CO2 and melamine (m/z, 126) are observed (Fig. 5b). undertake sublimation route. On the other hand depolymerisation of

Table 3 Key chemical species identiﬁed from py/GC/MS by pyrolysis method.

Sample Pyrolysis Species identiﬁed (1C)

a b PA66/GF 420 H2O, CO2, cyclopentanone, 5-Hexenamine (m/z, 99), 5-aminopentonitrile, 1,6-hexanediamine, 1-methyl-3-formyl-indole (m/z,159), c1,8-diazacyclotetradecane-2,7-dione (m/z,226)

PA66/GF 500 H2O, CO2, 1-hexanamine (m/z, 101), cyclopentanone, hexane nitrile (m/z, 97), adiponitrile, 1,6-hexanediamine, N-(6-Aminohexyl)-4-pentenamide (m/z, 198), 1-hexene

PA66/GFþAlPiþS400 290 H2O, CO2,NH3, melamine, 1,8-diazacyclotetradecane-2,7-dione

(Successively) 350 CO2, Cyclopentanone, Melamine, 1,8-diazacyclotetradecane-2,7-dione 420 1,6-hexanediamine, 1-methyl-3-formyl-indole, 1,8-diazacyclotetradecane-2,7-dione

PA66/GFþAlPiþS400 500 H2O, NH3,CO2, Cyclopentanone, 1-hexaneamine, Hex-5-enylamine, melamine, 1,6-hexanediamine, 1-methyl-3-formyl-indole, 1,8-diazacyclotetradecane-2,7-dione

Fig. 6. FTIR of S400 and its degradation products. 52 A.D. Naik et al. / Fire Safety Journal 70 (2014) 46–60

ZnPO polyhedra leads to fragmentation or emergence of crystalline bands (broad) in this region (1157, 1094 and possibly overlapped phase of ZnPO. band at 1257 cm1) suggest amorphous ZnPO of ortho, pyro or There are three main de-ammonation and condensation pro- meta/polyphosphates origin. The 1257 cm1 band is assigned to 2 ducts of melamine–melam, melem and melon [40–42,45,46]. vas(PO2) and or v(P¼O) and the 1157 for vas(PO3 )andor 1 Tentatively, they are formed between 340–400, 400–450 and vs(PO2 ). The strong broad band around 926 cm indicate vas(P– 3 450–500 1C, respectively, with certain degree of flexibility in O–P) phosphate chain and possibly include v(PO4 ) whereas the 1 temperature range and shown to vary with experimental condi- low intense band at 739 cm is due to vs(P–O–P). The broad, tion [41]. Melam has been considered as the low temperature de- strong band around 510 cm1 is due to δ(P–O) [43,44]. ammonation product of melamine, which is formed upon linking The S400-750 spectrum is relatively simple as most of the two molecules of melamine with concomitant release of one mole organic part is removed and strong bands around 1068, 868, of ammonia. Melamine progressively condenses to give melam 501 cm1 corresponding to phosphate network with or without which then further fuses to form melem and via melon to g-C3N3 zinc coordination can be distinguished. Another noticeable feature [40–42,45,46]. S400 might pass through one or more of these in S400-750 spectrum is a broad distinct band around 2210 cm1 phases during its pyrolysis. possibly due to formation of a nitrile or cyanamide derivative The S400-376 spectrum logically represents transition of melam which is also observed in the pyrolysed product of MPP under to melem. There is drastic spectral change compared to neat S400 similar condition [39]. It was reported that if melamine is spectrum. The amine signals are broad, less intense and appear pyrolysed above 650 1C it partially cracked due to de-polymerisa- around 3125 and 3311 cm1. The shoulder band around 3400 cm1 tion, leading to cyanamide along with other products [41]. Further 1 disappears. The strong –NH2 band at 1673 cm is replaced by a at elevated temperature, heptazine framework may partly collapse medium broad band around 1642 cm1.Inaddition,newbands giving dicyandiamide, cyanamide and ammonia [41,45]. (1550 s, 1436 s, 1374 s, 1334 s, 1247 s) are identified. Pronounced absorption in this region is assumed to be due to v(C–N) and δ(N–H) 3.3.1.2. Degradation of formulations. FTIR on the residues of all the from more than one condensed products of melamine. These formulations are presented in Fig. 7. They have nearly similar pattern observations can be interpreted in line with de-ammonation fol- with broad, strong bands (except PA66/GFþS400) representing lowed by self-condensation leading to the formation of melamine amorphous nature of ZnPO and/or AlPO. The residue obtained from condensate-melam or coexistence of melam–melem. PA66/GFþS400 in tubular furnace (750 1C) is interesting as it shows v Spectral pattern changes from S400-376 to S400-554. The s/as relatively sharp bands corresponding to zinc orthophosphate type – v – fi 1 ( NH2), and ( NH) are signi cantly reduced and 1642 cm signal structure [43,44]. Although less prominent, same sample from mass prominent in S400-376 disappears and several overlapped bands loss calorimeter also exhibit slightly resolved IR pattern. This was 1 are identified (1618, 1555, 1410, 1318, 1250 cm )assignedto attributed to polymer mediated formation of crystalline ZnPO. In melem dimer/oligomers or melon type species or mixed species. most of the char forming materials the low intense band around 1 The bands between 1318 and 1250 cm are of C–Nstretchingand/ 750 cm1 is due to C–O–PorC–Pvibration. or N–H bending vibrations characteristic of the C–NH–Cunitin melem. The triazine ring bend is also shifted to 805 cm 1 suggest- 3.3.2. Solid state NMR characterisation of condensed phase analysis ing that material consists of non-protonated triazine or heptazine 3.3.2.1. Pyrolysis of S400 additive. DegradationinS400canbe building blocks [41]. It should be reminded from TG curve (Table 2) conveniently tracked by solid state NMR thanks to the presence of that S400 displays a very gradual weight loss in the second step of NMR sensitive nuclei in both inorganic (31P of ZnPO) and organic decomposition (430 1C onwards) which possibly involve formation components (13C of melamine) as they provide information [46–54] of multiple melamine condensation products. about fragmentation in phosphorus polyhedra and melamine In addition to above spectral changes, there are indications of condensation/degradation, respectively. 31PMAS-NMRofS400and changes in the composition of ZnPO component. The asymmetric its pyrolysed products are displayed in Fig. 8a. For the 31Pisotropic unresolved envelope between 1257 and 926 cm 1 is specificregion chemical shift of metal phosphates, there is a trend to lower values for ZnPO of varying degree of polymerisation [43,44]. The multiple with increase in the number of bridging oxygen atoms attached to a

Fig. 7. FT-IR (expanded region) on selected residues. A.D. Naik et al. / Fire Safety Journal 70 (2014) 46–60 53

counterparts α-Hopeite (α-Zn3(PO4)2 4H2O) and β-Hopeite (β- Zn3(PO4)2 4H2O) are reported between 4.5 and 4.3 ppm which appeared as a sharp singlet [54]. It is also reported that zinc

hydrogen phosphate (Zn3(HPO4)3 3H2O) show multiple signals around þ2.3, þ5.5, and þ6.5 ppm [54]. In S400-376, doublet around 21.6 and 24.4 ppm found in S400 is replaced by a broad band (centred at 24 ppm) and signals between 10 to 2 ppm decreases considerably. This indicates collapse of crystalline framework of ZnPO component of S400 suggesting depolymerisation. The set of signals between þ2.3 to þ6.7 ppm remains unchanged indicating that these are independent moieties. In S400-554 spectrum, the metaphosphate signals around 24 ppm decreases and broadened. Pyropho- sphate signals are still detected but they are broadened. Interest- ingly, signals between þ2.3 and þ6.7 ppm remain unaltered. In S400-750 spectrum there is a broad signal around 0 ppm indicating formation of amorphous phase possibly mixture of species including discrete ortho-phosphoric acid (Fig. 8a). 13C NMR spectrum of S400 (Fig. 9a) shows two asymmetric bands at 164.4 and 157.5 ppm assigned to carbon atoms of triazine ring. Due to tautomeric forms, melamine can exist in two forms and gives arise to two signals [21]. They have undergone changes in terms of intensity ratio upon annealing and also broadened indicating presence of multiple species due to de-ammoniation leading to condensation products of melamine. The down ﬁeld signal (163.9 ppm) is considerably broadened in S400-376 (163.9, 156.4 ppm). It is likely that it may incorporate signals for condensed molecules like melam or melem or even melamine–melam adduct. Since melam was found to have chelating ability as shown by several melam-metal complex [41], it is reasonable to assume that zinc vicinity stabilises this intermediate. Conversion of melamine to any condensation product changes the electronic equiva-

lence of carbon in the triazine ring (CN2(NH)x and CN3 moieties) and thus separate resonance would appear. Since they are hardly separable, the signals are very broad. Evidences about these species were discussed in preceding FTIR section. 13C NMR spectrum of S400-554 shows further broadening of 13C NMR signals (163.8, 155.6 ppm) and there is noticeable decrease of high-field side signal (155.6 ppm). At this stage condensation of melem sets in and oligomeric or polymeric heptazine based materials like melon or even small amount of its proton lacking Fig. 8. 31P MAS NMR spectra on (a) neat S400 and its degradation products, physically mixed AlPiþS400 degraded at different temperature. Here number counterpart graphitic carbon nitride (CNx) would exist. No sig- corresponds to pyrolysis temperature in tubular furnace. (b) 31P MAS NMR spectra nificant signal was detected in the spectrum of S400-750. on degradation samples of formulations from mass loss calorimeter. Numbers 100, As the S400 degradation materials are collected based on 300, 600, and 1000 are time (in second) at which the degrading polymer plates temperature of maximum degradation in TGA curve it is likely that fl n ‘ ’ under the heat ux are dismounted. Symbol ( ) denotes spinning side band. Neat the samples are mixture of products rather than single phase of any corresponds to undecomposed sample. melamine condensate. Also thermal treatment time duration (3 h) may not be sufficient for complete transformation into a single phosphorus atom [54]. The HPDEC (High power proton decoupling) species. For these reasons distinct identification of de-ammonation MAS NMR spectrum of S400 shows, an asymmetric doublet at 21.6 and condensation products from NMR or FTIR is tentative. and 24.4 ppm representing the Q2 tetrahedra, possibly at different crystallographic position and are the bases for meta/polyphosphate chains. There are several intense peaks between 10 to 2ppm 3.3.2.2. Degradation studies in manually mixed AlPiþS400. 31P MAS (10.7, 7.4, 6.5, 5.5, 2.2). This feature is due to Q1 sites that NMR of manually mixed and pyrolysed samples of AlPi and S400 are are either chain terminals or found in isolated pyrophosphate dimers. shown in Fig. 8a. In AlPiþS400-421 spectrum, it can be seen that most

α-Zn2P2O7 (15.9, 19.1, 21.2 ppm) and γ-Zn2P2O7 (5.5, 7.1, of the pyrophosphate in S400 have transformed and a broad signal 10.4 ppm) are reported to be found around the same region [52]. appears around 25 ppm similar to S400-376. The AlPi doublet in A third set of sharp isotropic peaks, are found at þ6.7, þ6.4, AlPiþS400-535 spectrum is completely disappearedandbroadsignals þ3.9, þ3.1 and þ2.3 ppm. CP measurement (Fig. 8a) shows that around 25 ppm increases in intensity with a shoulder peak at these signals (þ2.3 to þ3.9 ppm) disappear indicating proximity 9 ppm. The signals between þ2.2 to þ6.6 ppm remain nearly of this phosphorus tetrahedra to a proton source like melamine unaltered as in S400. The reason for the enhancement of signal moiety. These can also be discrete molecules or part of small poly- around 25 ppm is due the transformation of AlPi into AlPO4 and meric network with or without zinc coordination. There are few overlapping with phosphate polyhedra from zinc phosphate. The ZnPO molecules whose chemical shifts falls in this range. Roming representative spectra (27Al) from annealed AlPiþS400 are depicted in et al. [52] reported signals for α-Zn3(PO4)2, around þ3.9 ppm and Fig. 10 which shows presence of two broad signals around þ38 ppm two signals at þ7.6 and þ2.8 ppm for β-Zn3(PO4)2. Their hydrated and 12 ppm. The þ38 ppm is due to AlPO4 and the 12 ppm is due 54 A.D. Naik et al. / Fire Safety Journal 70 (2014) 46–60

Fig. 9. (a) Expanded region around aromatic carbon in 13C CP MAS NMR of neat S400 and pyrolysed products of S400. (b) 13C MAS NMR spectra on S400 and some selected formulations. Also shown are char formation in the residue.

Fig. 10. 27Al MAS NMR spectra on degradation samples of PA66/GFþAlPiþS400 from mass loss calorimeter. 100, 300, 600, and 1000 are time (in second) at which the degrading polymer plates under the heat ﬂux are dismounted. Representative spectra of PA66/GFþS400-1000 and manually mixed samples (AlPiþS400) at 421, Fig. 11. (a) XRD pattern for S400 and its pyrolysed samples (b) XRD pattern on and 535 1C carried out in tubular furnace are also shown. selected residues obtained in tubular furnace (750 1C) and mass loss calorimeter (1000 s). to undecomposed AlPi. Presence of a weak band around 10 ppm indicates AlO5(PO) intermediate in the formation of AlPO4. pyrophosphate signal is more prominent than other signals in PA66/GFþS400. This trend increases progressively from PA66/ GFþS400-100 spectrum to PA66/GFþS400-300 spectrum indicat- 3.3.2.3. Degradation studies in PA66/GFþS400. 31P NMR of PA66/ ing fragmentation. Further in PA66/GFþS400-300 spectrum, signals GFþS400 and its degradation products collected based on HRR are broad and there is an additional broad signal at 30 ppm. The curve are shown in Fig. 8b. Compared to neat S400 spectrum, PA66/GF þS400-600 spectrum displays signiﬁcant changes with a A.D. Naik et al. / Fire Safety Journal 70 (2014) 46–60 55

Fig. 12. PA66/GFþAlPiþS400 sample from mass loss calorimeter: (a) BSE image on shiny intumescence surface, (b) optical microscope image on upper surface of char layer showing random distribution of white islands. (c) BSE image on white lumps seen on image (b). ((d)–(g)) Elemental mapping (N, Zn, P and Al respectively) on intumescence surface. weak broad signal around 10 ppm and sharp intense peaks at crystalline nature of products. In addition less intense signals are 26.2 ppm and 35.2 ppm. The 10 ppm signal is due to also present around þ5 ppm due to zinc orthophosphate. fragmented pyrophosphate species whereas 26.2 and 35.2 ppm are assigned to re-structured crystalline form of zinc polyphosphate. 3.3.2.4. Degradation studies in PA66/GFþAlPiþS400 formulation. In The 13CNMRspectrumofPA66/GFþS400 (Fig. 9b) shows signals that 31P MAS NMR spectrum of PA66/GFþAlPiþS400 (Fig. 8b), an intense corresponds to carbon source of PA66 whereas signals from S400 are symmetric doublet from AlPi is observed at 43.6 and 41.6 ppm, very weak. Multiple peaks between 42.1 to 25.4 ppm are seen whereas weak signals are seen for S400 and this pattern is retained indicating –CH2 groups of PA66 and the carbonyl function appear at in PA66/GFþAlPiþS400-100 spectrum. In PA66/GFþAlPiþS400- 173.3 ppm. Additive insertion into PA66/GF increases the overall 300 spectrum, AlPi signals decrease and appear as asymmetric thermal stability of PA66/GF and thus decomposition of PA66/GF is doublet which mark the onset of its decomposition and there delayed and signals corresponding PA66/GF is still seen in PA66/ appear also a very broad weak signal around 30 ppm. This signal

GFþS400-300 spectrum (not shown). At PA66/GFþS400-600, a broad is mainly due to zinc phosphate but may also include AlPO4 as weak resonance around 130 ppm is seen which is assigned to char beginning of AlPi decomposition can contribute to it. AlPi signals formation. disappear in PA66/GFþAlPiþS400-600 spectrum and the broad In PA66/GFþS400-468 (from tubular furnace) pyro- (11.2 ppm) signal around 30 ppm is prominent which further increases in and metaphosphate (25.3 ppm) have equal proportion compared intensity in PA66/GFþAlPiþS400-1000 spectrum. This broad signal to dominant pyrophosphate in PA66/GFþS400. Further in PA66/ may have contribution from stable species like AlPO4 and ZnPO or GFþS400-578, there is major transformation into meta/polypho- even mixed phosphates. sphate (30.1 ppm as unresolved doublet) which also corresponds Spectrum of 27Al MAS NMR of PA66/GFþAlPiþS400 shows to completion of 3rd step of degradation in TGA. Small amount of two signals (Fig. 10), a broad resonance around 50 ppm and a pyrophosphate present appears as multiplet between 15.8 and sharp peak at 12.3 ppm. These are respectively assigned to 7.1 ppm. These signals are relatively sharp and may indicate alumina in glass ﬁbre and octahedral aluminium of AlPi [26,29]. 56 A.D. Naik et al. / Fire Safety Journal 70 (2014) 46–60

The PA66/GFþAlPiþS400-100 spectrum shows broadening of low Zn2(P2O7). It has been observed that polymer medium often acts as field signal with the appearance of two maxima indicating presence asacrificial matrix to generate crystalline, phase pure ZnPO. of another species. This additional signal is more prominent in PA66/GFþAlPiþS400-300 and very distinct in PA66/GFþAlPiþ 3.3.4. Texture studies S400-600. This species is due to formation of AlPO4 and AlPi The intumescence surface, underlying char and its upper surface disappears completely at this stage. No further change is observed were investigated by EPMA to determine the constituents of the 13 in PA66/GFþAlPiþS400-1000 spectrum. C NMR of PA66/GFþ char. This also shows the migration and distribution of chemical AlPiþS400 (Fig. 9b) shows signals that corresponds to carbon of species under fire scenario. The BSE image on intumescence is PA66 and AlPi. Spectrum is similar to that of PA66/GFþS400 but shown in Fig. 12a and corresponding elemental mapping are shown with additional signal for AlPi which shows a multiplet between in Fig. 12d–g. There is random distribution of white granules 22.4 and 20.4 ppm and a strong peak at 8.6 ppm assigned respec- throughout the surface. The needle shaped structures seen at certain – – tively to CH2 and CH3 groups. Signals of PA66/GF are still found in places are glass-fibres. EPMA analysis confirms that the surface is the spectrum of PA/GFþAlPiþS400-300 indicating thermal stabili- rich in nitrogen, oxygen, carbon, zinc, phosphorus, and aluminium zation of PA66 by additives. In PA66/GFþAlPiþS400-600 spectrum, distributed over the entire surface. There is hardly any difference in a broad weak resonance around at 130 ppm is seen which is terms of elemental composition between white patches and the assigned to char formation [26]. surrounding dark matrix. Further, it is observed that the upper surface of underlying char also display random distribution of white lumps of 2–3mm width (Fig. 12b). Same elements that were mapped on intumescence surface are detected except carbon. 3.3.3. X-ray powder diffraction studies (XRD) A cross-section on the char layer is shown in Fig. 13a, which shows XRD pattern of selected residues were displayed in Fig. 11.XRD a highly porous structure. A 3D image constructed from optical fi of S400 is well de ned indicating crystalline nature of the material. microscope (Fig. 13b) shows numerous interconnected pores. A BSE 1 1 1 As the temperature increases (376 Cto554Cto750C) the image over one such area is shown in Fig. 13c and an elemental diffraction peaks weaken due to amorphozation. The XRD pattern mapping performed over this surface are shown in Fig. 13d–f. The 1 of S400-750 shows a broad band around 25 due to char formation. skeletal framework is constructed from glass fibres, carbon and all It is found that ZnPO framework has a pronounced tendency to the key elements like P, Al and Zn making a robust solid sponge. The collapse upon removing the amine template from the framework by voids that can be seen in the images are assumed to be developed dry heating unlike AlPO. It is also possible that presence of acid during burning which acts as escape channels for gases. source like H3PO4 lead to degradative transformation of ordered molecular network in S400 into low dimensional amorphous solids [53]. The residues obtained from formulation involving AlPi also 4. General discussion display broad band due to char formation. Although few sharp diffraction peaks are seen in AlPiþS400-750, assigning to specific Under fire scenario virgin PA66 due to its inherent flammability crystalline phase is difficult as other peaks are overlapped in the burns easily. Despite its rich carbon framework which could be broad band from char. Interestingly, residue from PA66þS400-750 source of char it fails to build sufficient stable cross-linking and obtained from tubular furnace under controlled condition shows subsequent thermally stable char. Adipic acid based polymers like well defined diffraction pattern. Comparison with the literature and PA66 were proposed to thermally degrade via a H-transfer to the database search [14,53–58] revealsthatitisamixtureofseveral nitrogen atom with formation of compounds having amine and species like dehydrated form of α-hopeite (Zn3(PO4)2, Zn(PO3)2 and keto-amide ends groups (Scheme 1). Successively, secondary

Fig. 13. Char layer from PA66/GFþAlPiþS400 obtained from mass loss calorimeter: (a) optical microscope image showing cross sectional view on char, (b) 3D view on porous char. As a guide, dark blue color indicates presence of cavities leading to internal channels created from escaping gases. (c) BSE image on cross-section of char embedded in epoxy resin. White spots are glass ﬁbres whereas grey areas represent presence of other elements mixed with carbon skeleton. ((d)–(f)) Elemental mapping (Zn, P and Al) on image (c) showing distribution of elements. (For interpretation of the references to color in this ﬁgure legend, the reader is referred to the web version of this article.) A.D. Naik et al. / Fire Safety Journal 70 (2014) 46–60 57

thermal fragments leads to compounds like cyclopentanone, CO2, In the preceding section, it is noted that synergy between S400 compound with azomethine and isocyanate. Cyclopentanone, a and AlPi has improved flame retardancy of PA66/GF (Fig. 3a) volatile product is shown to result from cyclization of adipic acid in PA66/GFþAlPiþS400. Otherwise susceptible PA66/GF shows portion. When this happens at adipic acid terminated chain end of 80% reduction in rate of heat release in mass loss calorimeter polyamide 66, there is concomitant release of carbon dioxide. and moreover the ignition time is significantly prolonged (111 s). It Literature reveals that there may be several types of competitive is evident from pyrolysis GCMS that melamine is released from reaction in the thermal degradation of PA66 and additives often S400 at an earlier stage that might trigger destabilization of found to influence this degradation pathway in a favourable way polyamide network (Scheme 2). The other potential species are [26,27,30–33,37,59,60]. ammonia, a de-ammonation–condensation side-product of S400 that is detected in TGA-FTIR performing the same role. In the following section, the identifications of key intermediates that were discussed in the previous sections are gathered and results are converged to frame a tentative sequence in flame retardancy mechanism. This covers both gas and condensed phase actions with remarkable physical response of protective intumescence. The first obvious and visual indication of flame retardancy is intumescence. Only AlPiþS400 combination offers a significant intumescent phenomenon to PA66/GF (Fig. 3c). The ‘breathing effect’ of swelled char largely regulates the increasing pressure created by the gases underneath thereby regulating control release of flammable/non-flammable gases. This also acts a physical barrier against external heat flux. Since both S400 and AlPi have high probability of sublimation, it also contributes to swelling of intumescence due to pressure developed underneath an action called blowing effect. This effect might bring some elements to the intumescence surface contributing to the dense coating of elements. The intumescence is vitreous, strong and flexible enough to withstand gaseous pressure thanks to the glass-fibres as at no fi Scheme 1. A simplified, tentative degradation pathway and decomposition pro- point of time the hollow intumescence collapses. EPMA con rms ducts in PA66/GF. that all the key elements (P, Al, Zn, N, C, O) that were part of the

Scheme 2. Tentative degradation pathway and prominent decomposition products in the gas phase action of PA66/GFþAlPiþS400. 58 A.D. Naik et al. / Fire Safety Journal 70 (2014) 46–60 additives/polymer were found on the surface of intumescence. The thermal degradation. S400 alone is found to have higher tendency to dispersion of these vitreous-glass forming materials cement the form low dimensional phosphate products like ortho/pyro phos- intumescence and along with densely interwoven glass-ﬁbre gives phates (ZnPO). On the other hand polymer mediated decomposition cohesiveness and robustness to intumescence. It is interesting to shows formation of crystalline meta/polyphosphate chains (Fig. 8b). note that 3 mm thick polymer plate under mass loss calorimeter The source of identiﬁed AlPO4 is AlPi and its evolution is traced: experiment develop 4 cm hollow intumescence (Fig. 3c) provid- (AlO6(–PO) to AlO5(–PO) to AlO4(–PO)) [29] and obviously ZnPO is ing protection to the underlying polymer. obtained from S400. Although amorphoziation is evident from XRD

Alongside, the underlying polymer degrades to form a porous pattern, dehydrated form of α-hopeite (Zn3(PO4)2,Zn(PO3)2 and char (Fig. 13). Analysis on collected condensed phases based on HRR Zn2(P2O7)isidentified underlining the role of polymer matrix in curve is a trail of sequence of events that had taken place during the shaping the crystallinity of zinc phosphate. The upper surface of char course of thermal degradation of flame retarded polymer. Elemental bears interesting islands of white lumps whose elemental identifica- mapping by EPMA (Fig. 13)confirms presence of all elements (P, Al, tion indicates all the expected elements except carbon. This probably Zn,N,C,O)embeddedincarbonmatrix.Theseelementsalongwith indicates some sort of migration and accumulation of elements. The glass fibres reinforce the char layer. PA66 is the main source of carbon role of phosphorus-containing compounds in flame retardancy in partly contributed from S400 and AlPi. Solid state NMR was useful as condensed phase is multi-modal. They are known to be involved in true molecular identities of the elements in char could be established branching and cross-linking of decomposition products of polyamide (Section 3.3.2) that are formed at different steps in the course of thus reducing combustible gaseous products [27]. Their role in

Scheme 3. Tentative degradation pathway and decomposition products in the condensed phase action of PA66/GFþAlPiþS400. A.D. Naik et al. / Fire Safety Journal 70 (2014) 46–60 59 formation of inorganic glasses is well documented. They are also S400 is a tailored multi-component molecular system with each found to obstruct the gasification of char. It could be expected that component capable of contributing to flame retardancy. S400 char/intumescence further undergo oxidation in air at elevated influence processability, improves color of extrudate, shows syner- temperature ruining its significant contribution. FTIR indicates that gism with additives, and enhances fire performances. A detailed there may be formation of C–PorC–O–Pbonds(Scheme 3)which investigation on how S400 is able to execute itself in the might explain the stability of char layer towards oxidation. It was strengthening of polyamide 66 towards flame retardancy is extra- reported by McKee et al. [61,62] that carbonaceous materials treated polated in this work. The inclusive aspects extracted in this work with ortho-phosphoric acid lower their oxidation rate because raising to delineate the mechanism of action in flame retardancy would the temperature results in the decomposition of the adsorbates and be an impetus for further redesign strategy for novel metal based the formation C–O–Pbonds.Hence,carbonactivesitesareblocked melamine polyphosphates. and oxidation of carbon is limited. Often such phosphate glasses were incorporated with metal ions like aluminium, zinc or zirconium References to enhance performance [61–63]. Independent studies on S400 degradation points to the forma- [1] S. Bourbigot, M. Le Bras, J. Troitzsch, Fundamentals: introduction, in: J. Troitzsch tion of conjugated aromatics upon de-ammonation. There is visual (Ed.), Flammability handbook, Hanser Verlag, Munich, 2003, pp. 3–7. indication of chromic response upon annealing. White S400 [2] C.A. Wilkie, A.B. 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