Flame retardancy mechanisms of aluminium phosphinate in combination with melamine polyphosphate and zinc borate in glass-fibre reinforced polyamide 6,6

Polymer Degradation and Stability 92 (2007) 1528e1545 www.elsevier.com/locate/polydegstab

Flame retardancy mechanisms of aluminium phosphinate in combination with melamine polyphosphate and zinc borate in glass-fibre reinforced polyamide 6,6 Ulrike Braun a, Bernhard Schartel a,*, Mario A. Fichera b, Christian Ja¨ger b b

a Federal Institute for Materials Research and Testing, BAM, Unter den Eichen 87, 12205 Berlin, Germany Federal Institute for Materials Research and Testing, BAM, Richard-Willstaetter Str.11, 12489 Berlin, Germany

Received 22 February 2007; received in revised form 13 April 2007; accepted 13 May 2007 Available online 21 May 2007

Abstract The fire retardancy mechanisms of aluminium diethylphosphinate in combination with melamine polyphosphate and zinc borate was analysed in glass-fibre reinforced polyamide 6,6. The influence of phosphorus compounds on the polyamide decomposition pathways was characterized using thermal analysis (TG), evolved gas analysis (TGeFTIR), and FTIReATR analysis of the residue. The Lewis acidebase interactions between the flame retardants, the amide unit, and the metal ions control the decomposition. The flammability (LOI, UL 94) and performance under forced-flaming conditions (cone calorimeter using different irradiations) were investigated. Fire residues were analysed with FTIReATR, SEMeEDX, and NMR. Aluminium phosphinate in polyamide 6,6 acts mainly by flame inhibition. Melamine polyphosphate shows some fuel dilution and a significant barrier effect. Using a combination of aluminium phosphinate and melamine polyphosphate results in some charring and a dominant barrier effect. These effects are improved in the presence of zinc borate due to the formation of boronealuminium phosphates instead of aluminium phosphates. Ó 2007 Elsevier Ltd. All rights reserved. Keywords: Flame retardancy; Polyamide 6,6; Metal phosphinate; Melamine polyphosphate; Zinc borate

1. Introduction The flame retardancy of glass-fibre reinforced polyamides used in electrical and electronic equipment has been a topical challenge for a long time. The possibilities are manifold. Effective solutions have been found for derivatives containing halogen, nitrogen and phosphorus [1e3]. Nowadays advanced halogen-free flame retarded systems are one of the most popular topics of relevant materials research and development. The most promising approaches are based on organic or inorganic compounds that contain phosphorus. The recently developed and commercialized metal phosphinates belong to a novel class of phosphorous flame retardants. Despite their

* Corresponding author. Tel.: þ49 30 8104 1021; fax: þ49 30 8104 1027. E-mail address: [email protected] (B. Schartel). 0141-3910/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.polymdegradstab.2007.05.007

increasing use, there is a lack of scientific understanding as to their fire retardancy mechanism, especially in different kinds of polymeric materials. Previous investigations show that phosphorus can act in both, the condensed and the gaseous phase [4e7]. In the gaseous phase, phosphorus results in flame inhibition through radical trapping; in the condensed phase, it initiates the formation of carbon char or inorganic residue. The activity of phosphorus depends on the flame retardant used as well as on the chemical structure of the polymer matrix and on interaction with other additives. The functional group of polyamides presents various possibilities for interactions between the polymer and the flame retardant [8]. For red phosphorus in glass-fibre reinforced polyamide, a mainly solid-phase mechanism was reported [9,10]. It was proposed that the oxidation of phosphorus is influenced strongly by moisture or other additives in the polymer [11]. Polyphosphates, such as ammonium or

U. Braun et al. / Polymer Degradation and Stability 92 (2007) 1528e1545

melamine polyphosphate in polyamides 6,6, also remain in the residue and form polyphosphate esters with the polymer, resulting in increased char formation [12e14]. This reactivity is also established for polyamide 6 [15e17] and is the basis of the intumescing formulations in other polymers [18,19]. For the stabilisation of char the presence of glass fibres can be crucial [13]. Aluminium phosphinates in glass-fibre reinforced polyamide 6,6 is satisfactorily effective only on rather high loadings of 30 wt% [20]. The phosphinate content can be reduced to about 10 wt% in the presence of melamine polyphosphate [21]. Synergistic or catalytic effects are known when metal salts and polyphosphate compounds are used [5,7,22e24]. First investigations about aluminium phosphinate and melamine polyphosphate as flame retardants in poly(methyl methacrylate) showed that the fire retardancy mechanism is mainly based on a condensed phase action [25]. However, the decomposition mechanism of the additives alone and the additives in polymer is not discussed in detail. Beyond this effect, the efficiency can be improved by the addition of boron compounds to phosphorous flame retardants [26,27]. In this work the pyrolysis and fire behaviour are investigated with respect to the fire retardancy mechanisms in a glass-fibre reinforced polyamide 6,6 that is flame-retarded with aluminium phosphinate (AlPi), melamine polyphosphate (MPP) and zinc borate (ZnB) (Fig. 1). For a systematic description and understanding, different combinations were investigated, including glass-fibre reinforced polyamide 6,6 samples with (i) no flame retardant, (ii) AlPi, (iii) MPP, (iv) AlPi þ MPP, and (v) AlPi þ MPP þ ZnB. This paper not only comprehensively illuminates the fire retardancy effect of these materials, but also gives insight into the pyrolysis of the materials. The focus is on the chemical reactions of phosphorus compounds with the polymer matrix and resulting decomposition pathways. 2. Experimental 2.1. Samples Five different materials based on polyamide 6,6 including 30 wt% glass fibres (subsequently shortened to PA66/GF) were investigated: PA66/GF, PA66/GF þ AlPi, PA66/ GF þ MPP, PA66/GF þ AlPi þ MPP, and PA66/GF þ AlPi þ MPP þ ZnB. A total flame retardant loading of 18 wt% was

Fig. 1. Chemical structures of AlPi and MPP.

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used in the latter case, with AlPi and MPP as major components and ZnB as a minor additive. The materials were prepared on a 27 mm twin-screw extruder using Ultramid A3 (BASF), Exolit OP 1230 (Clariant) and Melapur 200/70 (Ciba). Samples were provided by Clariant Produkte (DE) GmbH in granulate, bars for the UL 94 and LOI tests as well as plates for cone calorimeter investigations. 2.2. Thermal analysis Thermogravimetric (TG) experiments were performed using TGA/SDTA 851 (Mettler Toledo, Gießen, Germany) with a nitrogen or synthetic air flow of 30 ml min1. The samples (about 10 mg) were heated in 150 ml alumina pans with cups, from room temperature up to about 1200 K at heating rates of 1, 2, 5, 10 and 20 K min1. The cups were used in order to improve the reproducibility, especially with respect to small and overlapping decomposition steps. The cups did not prevent the expansion of samples, because the sample size was an order of magnitude smaller (w10 ml) than the cups. The TG was coupled with a FTIR Spectrometer Nexus 470 (Nicolet Instruments, Offenbach, Germany). A transfer tube with an inner diameter of 1 mm (heated to 523 K) connected the TG and the infrared cell (heated to 533 K). The infrared spectrometer, equipped with a DTGS KBr detector, was operated at an optical resolution of 4 cm1. The samples (about 15 mg) for the coupled experiments were heated in alumina pans without cups, from room temperature up to about 1200 K at heating rates of 10 K min1. Single spectra were chosen to identify the gases evolved. The identification was based on characteristic peaks indicating chemical compounds, and on comparison with reference spectra taken from a database. Product release rates were evaluated using the height of product-specific peaks as a function of time. Solid residues collected during TG at heating rates of 5 K min1 were investigated by means of FTIR spectroscopy (FTIR Spectrometer Nexus 670/870 by Nicolet Instruments) using the Smart Golden Gate Single Reflection Diamond ATR accessory. The spectra were taken with a DTGS KBr detector with an optical resolution of 4 cm1. The lower heating rate compared to TGeFTIR measurements was used in order to separate clearly the overlapping decomposition processes. 2.3. Fire testing The flammability of the materials was determined using the UL 94 classification according to IEC 60695-11-10 and the limited oxygen index (LOI) according to ISO 4589. Specimens with a size of 12.5  3  125 mm3 were used for both tests. Hence, the geometry of the bars was not the common specimen size for LOI measurements; however, these dimensions were sufficient to ensure reasonable LOI results [28]. The fire behaviour for flaming conditions was characterized using a cone calorimeter (Fire Testing Technology, East Grinstead, UK) according to ISO 5660. This method enabled investigation with respect to heat release rate (HRR), total heat release (THR), mass loss rate (MLR), time to ignition (tign),

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Fig. 2. TG and DTG of all samples, heating rate 10 K min1 (solid line ¼ PA66/GF, open circles ¼ PA66/GFeMPP, open squares ¼ PA66/GFeAlPi, solid squares ¼ PA66/ GFeAlPieMPP, solid rhombi ¼ PA66/GFeAlPieMPPeZnB).

rate of smoke release and carbon monoxide release rate. The integration of the release rates up to the end of test result in characteristics such as total heat evolved (THE), total smoke production (TSR), total carbon monoxide production (TCOR), and total mass loss (TML). The flame-out was defined as the end of test. Different external heat fluxes (35, 50, 70 kW m2) were applied. All measurements were repeated. All samples (specimen size: 100 mm  100 mm  5 mm) were measured in a horizontal position using a retainer frame to reduce unrepresentative edge-burning. The decreased sample area was taken into account for the calculations. The evaluation and discussion of cone calorimeter results were done according to the state of the art summarized in literature [29]. Residue analysis with FTIR spectroscopy was performed using a FTIR spectrometer (Nexus 470 by Nicolet Instruments) with the Smart Orbit Diamond ATR accessory. The spectra were taken using a DTGS KBr detector with an optical resolution of 4 cm1. Residue analysis was performed using scanning electron microscopy, SEM (FEI XL30, Fei, Eindhoven, the Netherlands) and energy dispersive X-ray spectroscopy, EDX (EDAX inc., Mahwah, USA). For EDX analyses, areas between 300 and 6000 mm2 were used and an acceleration voltage between 9 and 25 eV. 11 B and 27Al NMR spectra were acquired with an Avance 600 spectrometer (Bruker Biospin GmbH, Rheinstetten, Germany, B0 ¼ 14.1 T), whereas the 31P NMR measurements were performed on a Bruker DMX 400 spectrometer (B0 ¼ 9.35 T). Magic angle sample spinning (MAS) experiments were carried out at room temperature. 31P NMR spectra were collected at 161.9 MHz with a 4 mm double-resonance MAS NMR probe and a spinning frequency of 12 kHz. The 31 P single-pulse excitation measurements were performed using presaturation (saturation recovery technique [30] for checking the long spin lattice relaxation times) and proton dipolar decoupling using two-pulse phase modulation (TPPM) [31]. The 90 pulse length was 2.6 ms, and a repetition time of 256 s was sufficient to ensure full relaxation. 31P chemical shifts were referenced to 85% phosphoric acid (d ¼ 0 ppm)

using an external secondary standard (hydroxyapatite) set to d ¼ 2.3 ppm. The number of scans was 32. 11 B and 27Al spectra were recorded at 128.3 and 104.2 MHz, respectively, using a 4 mm MAS NMR probe with spinning frequencies of 10 kHz (11B) and 12.5 kHz (27Al). Pulse lengths of 1.5 ms with repetition times of 30 s for 11B and 2 s for 27Al were used in the single-pulse experiment without proton decoupling. 27Al and 11B chemical shifts were referenced to YAG (d ¼ 0.6 ppm) and to BPO4 (d ¼ 4.1 ppm), respectively. 100 scans were taken. 3. Results and discussion 3.1. Pyrolysis: decomposition behaviour The thermal decomposition of PA66/GF was characterized by a single decomposition step with maximum mass loss rate at 730 K. The resulting residue was of about 34 wt% and hence mainly consisted of glass fibres (Fig. 2, Table 1). When MPP was added, the decomposition behaviour changed significantly. Part of the decomposition process was shifted to a temperature of about 85 K lower with a maximum mass loss rate at 645 K, whereas the other part of decomposition remained unaffected at about 730 K. The residue at end of test was slightly increased to 37 wt%. Similar results were described in the literature for PA66/GF materials [10,12] and polyphosphate formulations in different kinds of polyamides [15,16]. The addition of AlPi to PA66/GF also influences the decomposition process, but not as much as MPP did. The first decomposition step was shifted to decomposition temperatures only about 40 K lower. The thermally stable residue at 1175 K was increased slightly to 36 wt%. When MPP and AlPi were combined in PA66/GF, additional decomposition steps occurred between 510 and 640 K. The maximal mass loss rate temperatures of the main decomposition processes of PA66/GFeAlPieMPP (695 and 730 K) agreed with the ones of PA66/GFeAlPi (690 and 720 K). However, for PA66/GFeAlPieMPP no distinct separation of the decomposition processes was possible. Compared to PA66/GF, the residue at the end of TG experiment was clearly

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Table 1 TG analysis, based on TGeFTIR investigations, heating rate 10 K min1 PA66/GF

PA66/GFeMPP

PA66/GFeAlPi

Preceding mass loss Dweight/wt% DTGmax/K DT/K I. Main mass loss Dweight/wt% DTGmax/K DT/K

66.6  2 727  2 575e1175

II. Main mass loss Dweight/wt% DTGmax/K DT/K Residue at 1175 K Weight/wt%

33.6  2

PA66/GFeAlPieMPP

PA66/GFeAlPeMPPeZnB

8.5  1 602  2 510e640

4.6  3 631  2 575e640

39.5  1 644  2 575e680

42.4  3 688  2 575e710

35.9  4 695  2 640e720

36.2  5 683  2 640e710

24.4  1 731  2 680e1175

23.0  3 720  2 710e1175

15.2  4 (w728) 720e1175

21.3  2 722  2 710e1175

36.6  1

35.6  2

40.4  1

38.2  1

Averaged values, error of weight change based on maximal deviation.

increased by about 7 wt%. For the decomposition of PA66/ GFeAlPieMPPeZnB, a small preceding mass loss occurred around at 630 K. In contrast to PA66/GFeAlPieMPP, this process overlapped with the main mass loss steps at 683 and 722 K and cannot be separated when high heating rates were used. The residue at 1175 K increased to 38 wt%. The results of TG indicate a strong interaction between the additives containing phosphorus and the polymer. The interaction between the polyphosphate and PA66/GF induces a stronger shift in decomposition temperature than the interaction between the phosphinate and PA66/GF. When MPP and AlPi are combined, an additional preceding decomposition step occurs. With respect to the impact on the polymer decomposition, i.e. the separation of decomposition processes, AlPi is dominant, but not MPP. The stronger shift in decomposition temperature typical for polyphosphate did not occur in the combination of MPP and AlPi.

Above the main mass loss steps a minimal ethene formation (950 cm1) and diethylphoshinic acid release occurred. The decomposition of PA66/GFeAlPieMPP was accompanied by the release of diethylphosphinic acid with characteristic bands at 3650 cm1 (POeH), 1275 and 1243 cm1 (P]O, PeC) as well as 850 cm1 (PeO) (Fig. 3b) [35,36]. No vaporisation of AlPi was observed. The vibration energies of phosphorus bonds are partly different to the commonly known peak positions in solid state, especially the POeH bond vibration because in gas phase spectra the vibration energies are not affected by association effects [36, 37]. The product release rates illustrate the differences between all materials (Fig. 4). The comparison of product release rates with the DTG signal shows that the lower-temperature

3.2. Pyrolysis: evolved gas analysis The evolved gas analysis for PA66/GF exhibited characteristic bands of ammonia (965, 930 cm1), carbon dioxide (2354 cm1), cyclopentanone (1766 cm1), hydrocarbons (2960 cm1) and methane (3015 cm1), proving these to be the main decomposition products (Fig. 3a). The decomposition products are in accordance with the literature [10,12,32]. The flame-retardant materials showed the same decomposition products for polymer decomposition. An increased ammonia release was not found for MPP materials, according to the established condensation to melon phosphates derivates [34]. Although ammonia was identified unambiguously, its quantification was not possible, because it is also easily absorbed in the transfer line, forming ammonium salts. Additional decomposition products of MPP decomposition were not found, such as isocyanic acid and carbon dioxide [33]. For PA66/GFeAlPi and PA66/GFeAlPieMPPeZnB, vaporisation of AlPi was observed. The spectrum of the AlPi molecule is characterized 1 by signals of PO (Fig. 3c) [35]. 2 anion at 1147 and 1083 cm

Fig. 3. Characteristic evolved gas analysis spectra of (a) PA66/GF at 43 min with reference spectra of cyclopentanone (open circles) and hexanediamine (solid squares), (b) PA66/GFeAlPieMPP at 31 min and (c) PA66/GFe AlPieMPPeZnB at 42 min.

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Fig. 4. Product release rates of all samples measured by FTIR, during decomposition between 500 and 900 K (solid line ¼ PA66/GF, open circles ¼ PA66/GFe MPP, open squares ¼ PA66/GFeAlPi, solid squares ¼ PA66/GFeAlPieMPP, solid rhombi ¼ PA66/GFeAlPieMPPeZnB).

decomposition process (575e680 K) is accompanied by the release of cyclopentanone, CO2 and NH3, whereas the unaffected higher-temperature decomposition process (680e 770 K) leads to hydrocarbon release. The release of cyclopentanone was reduced and the release of CO2 was increased for PA66/GFeMPP as compared to PA66/GF. In the presence of AlPi the formation of cyclopentanone was reduced slightly and CO2 increased slightly (PA66/GFeAlPi, PA66/GFe AlPieMPP, PA66/GFeAlPieMPPeZnB). The evolution of amine derivatives and methane was not affected by additives. In accordance with the shift in decomposition temperature, there was a change in cyclopentanone and CO2 release. It is concluded that the shift in decomposition temperature for PA66/GFeAlPi results from the same interaction with the polymer, but being more moderate for AlPi than for MPP. Diethylphosphinic acid was produced from PA66/GFe AlPieMPP, especially during the preceding mass loss step, whereas in PA66/GFeAlPi and PA66/GFeAlPieMPPeZnB vaporisation the phosphinate salt occurred during the complete decomposition process. For both materials a small amount of phosphinic acid was released above the main mass loss steps. 3.3. Pyrolysis: residue analysis The decomposition residues were investigated with respect to the decomposition processes, i.e. residues were collected directly after characteristic mass loss steps. The initial spectra

of non-treated PA66/GF and PA66/GFeMPP are shown in Fig. 5a, spectra I. The band positions of PA66/GFeMPP correlate with the signals of PA66/GF: 3280, 3080 cm1 (n(NeH)); 2927, 2858, 1458, 1435, 1414 cm1 (n(CeH) and d(CeH)); 1629, 1530 cm1 (n(C]O) and n(NeH)). There was no clear difference between PA66/GF and PA66/GFe MPP with respect to the additive MPP, possible MPP signals are covered from PA66/GF spectrum. When the decomposition of PA66/GFeMPP was stopped at 660 K (Fig. 5a, spectrum II), the amine signals decreased (3290, 3080 cm1), as did the amide signals (1570, 1624 cm1), indicating the decomposition of the polyamide structure. However, significant amounts of hydrocarbon structures remained (2915, 2848, 1455, 1435 cm1). In the fingerprint area characteristic bands developed (1260, 1078, 970 cm1), which were caused by phosphate and phosphate esters [35,16]. Treating the PA66/ GF and PA66/GFeMPP materials at 770 K resulted in very similar spectra (Fig. 5a, spectrum III). The spectra were dominated by broad signals of glass fibre at 1390 and 900 cm1. The small shoulder for PA66/GFeMPP at 1075 cm1 was attributed to phosphorous ester vibrations [35,16]. Characteristic PeOeP vibration frequencies around 900e1000 cm1 were masked from the glass-fibre signal. The PA66/GFeAlPi spectrum consisted of a PA66/GF spectrum with some additional signals (Fig. 5b, spectrum I). The additional bands localized at 1144 and 1070 cm1 corresponded with the most intensive signals of a spectrum of AlPi

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Fig. 5. FTIR spectra for initial phase (I), during decomposition (II) and after main decomposition (III) of (a) PA66/GF and PA66/GFeMPP, (b) PA66/GF and PA66/ GFeAlPi, (c) PA66/GF and PA66/GFeAlPieMPP, (d) PA66/GF and PA66/GFeAlPieMPPeZnB (see text for treatment temperatures).

itself at 1149 and 1075 cm1 (PO2). Analogous to the results for PA66/GFeMPP, the thermal treatment of PA66/GFeAlPi at 680 K resulted in the disappearance of the amide structure and remaining hydrocarbon structures (Fig. 5b, spectrum II). The bands characteristic for phosphinate remained at 1144 and 1066 cm1. After the main decomposition steps of PA66/GFeAlPi, again the broad signals of glass fibres were detected, but also additional characteristic bands of AlPi (Fig. 5b, spectrum III). Hence, part of the flame-retardant additive still existed at temperatures above 770 K. At the end of the TG experiment, at 1175 K, no phosphinate salt signals remained. The initial spectrum of PA66/GFeAlPieMPP is similar to that of PA66/GFeAlPi (Fig. 5c, spectrum I). This is not surprising, since MPP does not show any characteristic signals in PA66/GFeMPP, either. The thermal treatment at 610 K, which marks the end of preceding decomposition step, resulted in no differences in the polymer spectrum, but in a reduction in the bands characteristic for phosphinate (Fig. 5c, spectrum II). The final spectrum of PA66/GFeAlPieMPP (heated at 770 K) was characterized by glass-fibre bands and new broad signals around 1600 and 1100 cm1 (Fig. 5b, spectrum III). The band around 1600 cm1 was attributed to polyaromatics or carboneoxygen signals from PA66/GF residue

[37]. The 1100 cm1 signal is caused by aluminium phosphates, such as aluminium polyphosphate, aluminium pyrophosphate or aluminium orthophosphate, which was proved by reference measurements and literature data [38,39]. The exact position of the signal depends on the treatment condition: at 770 K it was localized at 1080 cm1, whereas at 1175 K the maximum of band was found at 1110 cm1. The shift is caused by a change in the composition of the mixture of the different aluminium phosphates. In the initial spectra of PA66/GFeAlPieMPPeZnB the same bands from phosphinate were observed as for PA66/ GFeAlPi and PA66/GFeAlPieMPP (Fig. 5d, spectrum I). However, after the preceding mass loss step at 680 K, the initial spectrum of PA66/GFeAlPieMPPeZnB was not changed significantly (Fig. 5d, spectrum II). The bands characteristic for phosphinate were still present and showed no significantly decreased intensity. Thermal treatment at 770 K resulted in broad bands from phosphinate at 1144, 1070 and 780 cm1, as well as a signal around 1040 cm1. The strong band around 1080 cm1 was not observed at this temperature; however, it was detected at the end of TG experiment at 1175 K. It was concluded that for PA66/GFeAlPieMPPeZnB the formation of different aluminium phosphates was not as dominant as for PA66/GFeAlPieMPP.

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Table 2 Calculated and experimental decomposition steps of major decomposition pathways for all PA66/GF materials In wt%

PA66/GFeAlPieMPP

PA66/GFeAlPieMPPeZnB

Preceding mass loss Et2P(O)OH Melamine Sum (calc.)

7.1(2) 3.5 10.1

3.5(1) 3.5 7.0

Sum (exp.)

9.2  0.4

I. Main mass loss Melamine Et2P(O)OH Cyclopentanone CO2 NH3 Sum (calc.)

PA66/GF

26.0 6.8 5.3

PA66/GFeMPP

24.1 12.6 4.9

3.5

3.5

23.9 6.3 4.8 38.5

22.1 11.6 4.5 41.7

37.9  1.0

Sum (exp.) II. Main mass loss AlPi Hydrocarbon Sum (calc.)

23.8 61.9

Sum (exp.)

66.7  0.9

28.4 70.0

21.9 21.9

34.0  1.3

26.1 26.1

26.0  1.0

36.6  0.8

21.8 5.7 4.4 31.9

20.2 10.6 4.1 34.8

38.2  2.5

Residue (sum includes additional 30 wt% glass fibres) eCNe 8.1 0.0 7.4 0.0 2.2 2.2 P-Comp. Phosphate Sum (calc.) 38.1 30.0 39.6 32.2 Sum (exp.)

PA66/GFeAlPi

11.4 20.0 31.4

11.4 23.8 35.2

18.0 18.0

3.5(1) 18.2 9.5 3.7 35.0

0.0

36.7

30.0

6.1 3.6 Al-phosphate 39.7 39.5  0.8

1.8(0.5) 19.3 5.1 3.9 30.1

1.8(0.5) 17.9 9.4 3.7 32.7

30.7  5.1

21.5 21.5

16.0  2.4

6.7

3.5(1) 3.5 7.0

8.4  3.7

35.7  2.8

28.1  2.2

34.3  1.1

3.5(1) 19.7 5.1 4.0 32.3

7.1(2) 3.5 10.1

5.7(0.5) 17.7 23.4

5.7(0.5) 21.1 26.8

23.4  1.0 0.0 3.6 33.6

6.1 3.3 B/Al-phosphate 39.2

0.0 3.4 33.3

39.8  1.2

Left column is attributed to thermal decomposition pathway, right column to hydrolytical decomposition pathway. Small brackets include the number of molecules of relative additive content; the presented experimental data are averaged about all measurements.

3.4. Decomposition models The following decomposition models were worked out in consideration of the described characteristics of the decomposition processes, such as characteristic temperature, mass loss, pyrolysis gases and changes in the condensed residue, and taking into account the interactions occurring between the compounds. The postulated decomposition models consider the major decomposition pathways and neglect minor decomposition pathways. The experimental data are based on the TG experiments with different heating rates, cupped alumina pans and without a coupled FTIR cell. So the TG curves show a more consistent behaviour, especially the small preceding mass loss steps. The minimal and maximal values of each curve are presented in Table 2. In accordance with the literature [12,31,40], two different decomposition pathways of PA66/GF were considered: the thermal and the hydrolytic polymer decomposition pathways (Fig. 6a, Table 2). Thermal decomposition results in a small residue amount of carbonenitrogen components, whereas hydrolytic decomposition is accompanied by no residue formation. The comparison with experimentally determined residues of 32e36 wt% indicates that both decomposition pathways took place. The hydrolytic decomposition is initiated by adsorbed water. Actually a realistic water content of 1.5 wt% in PA66/GF results in one water molecule on four PA66 monomer units.

The decomposition of PA66 was changed significantly in the presence of MPP (Fig. 6b, Table 2). The lowering of decomposition temperature indicated a catalytic effect of MPP on the amide scission. Hence, Lewis acidebase interactions were assumed. The phosphate is the hardest or strongest Lewis base, whereas the a-carbon at the amide bond is the hardest Lewis acid. Based on the Lewis acidebase concept [41] the formulation of such acidebase complexes results in an improved scission of the amide bond, and consequently in the enabled release of cyclopentanone, CO2 and NH3. The decreased cyclopentanone release is caused by a further Lewis acidebase interaction of cyclopentanone molecules with a-substituted cyclopentanone derivates [40], which condensed in the transfer line and thus prevented detection by FTIR. The release of hydrocarbons was not affected in the presence of a hard Lewis acid, because the chemical structure of the remaining hydrocarbons or amines provides no reasons for acidebase interactions. The MPP additive decomposed by ammonia release and subsequent melon formation [34] as well as melamine vaporisation and melamine decomposition [14,33]. In the decomposition model only the vaporisation of melamine is considered. It should be noted, that the model describes the main decomposition pathway, especially the main mass loss processes. The partly remaining of melon species influences the main mass loss at least about 2 wt%. The calculated mass loss of the first decomposition step (39e42 wt%) agreed with the experimentally determined residue of 37e39 wt%

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Fig. 6. Decomposition models of PA66/GF (a) and PA66/GFeMPP (b), PA66/GFeAlPi (c), PA66/GFeAlPieMPP (d), PA66/GFeAlPieMPPeZnB (e).

(Table 2). The same very good correlation was found for the second decomposition step, as well as the residue amount. This decomposition model of PA66/GFeMPP is also consistent with the decomposition behaviour of PA66/GF flameretarded with red phosphorus [10,11]. Assuming that the phosphorus in PA66/GF was oxidised to phosphate species, an analogous mechanism results in the very similar separation

of the decomposition processes of PA66 by adding red phosphorus. The decomposition model for PA66/GFeAlPi is presented in Fig. 6c and Table 2. Although most of the AlPi vaporises, a Lewis acidebase interaction with polymer is assumed. The phosphinate is a less oxidised phosphorus component than phosphate and hence it is a softer or weaker Lewis acid. The

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Fig 6. (continued)

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Fig 6. (continued)

interaction with the polymer is more moderate. Compared to PA66/GFeMPP the first decomposition process of PA66 is only slightly shifted to lower temperatures and both cyclopentanone and CO2 release are less affected. The calculated residue amount of 30e37 wt% fits very well with the experimentally determined values between 35 and 36 wt%. Comparing the data for the first and second decomposition step shows that the phosphinate vaporises during both steps. When AlPi and MPP were combined in PA66/GF, the pattern of the main decomposition steps agreed with the behaviour of PA66/GFeAlPi (Fig. 6d). The Lewis acid activity of the phosphate is suppressed by aluminium phosphate formation, because the aluminium ion is a harder Lewis acid than the a-carbon at the amide. Through formation of aluminium phosphate diethylphosphinic acid and/or melamine phosphinate are formed and released to the vapour phase during the preceding decomposition step. The step size shows that most of the phosphinate groups vaporise (w10 wt%). The calculated residue amount of 34e40 wt% fits well with the experimentally determined values between 38 and 39 wt%. The addition of zinc borate in PA66/GFeAlPieMPPeZnB changes the reaction of MPP with AlPi (Fig. 6e, Table 2). Now boron is the hardest Lewis acid and reacts with the phosphate. However, since the stoichiometric ratio of phosphate to boron is approximately 2:1, half of the aluminium also reacts with

the phosphate, vaporising melamine phosphinate before decomposition of the polymer. The lower rate of this reaction results in a smaller step size of the preceding decomposition step. The other half of the AlPi vaporises. The calculated residue of 34e40 wt% shows a good agreement with the amount determined by experiment (40 wt%). The zinc oxide in a strong substoichiometric ratio vanished by heating, it was concluded that zinc phosphinate was formed and vaporised. 3.5. Flammability and ignitability The flammability results are summarized in Table 3, including the ignition times from the cone calorimeter investigations. The HB classification of PA66/GF and PA66/GFeMPP as well as the LOI for PA66/GF (22%) and the increased LOI for PA66/GFeMPP (29%) agree with the values in the literature [1,2,13,42]. Although PA66/GFeAlPi achieved only a HB classification in UL 94 test, the LOI was increased impressively (38%). Higher loadings of aluminium salts are necessary to achieve a better UL 94 classification [20]. The discrepancy between improvement in LOI and the disappointing UL 94 resulted from a different residue performance in the two test geometries. In the LOI test the vertically positioned residue/char on top of the specimen remained during the test, whereas in the UL 94 test the residue fell off and exposed

U. Braun et al. / Polymer Degradation and Stability 92 (2007) 1528e1545

1538 Table 3 Flammability and ignition properties

UL 94 LOI Tign Tign TI.DTG a b

max

Irr.35 kW m2 Irr.50 kW m2 10 K min1 in air

PA66/GF

PA66/GFeMPP

PA66/GFeAlPi

PA66/GFeAlPieMPP

PA66/GFeAlPieMPPeZnB

%

1

HB 21.5

HB 28.9

HB 37.9

V-0 28.2

V-0 33.3

s s K

15 15 5

202 56 725

137 58 650

213 72 690

447a 82 710

eb 72 670

No homogeneous ignition. No visible ignition.

the untreated polymer material directly to the flame. Such influences of char properties on the UL 94 performance have been reported previously [43,44]. The LOI for PA66/GFeAlPieMPP (28%) and PA66/GFe AlPieMPPeZnB (33%) were improved compared to PA66/ GF. Both materials showed a moderate improvement in comparison to PA66/GFeMPP and an even worse result than PA66/GFeAlPi. However, PA66/GFeAlPieMPP and PA66/ GFeAlPieMPPeZnB achieved a V-0 classification in the UL 94 test. The burned residue did not fall off, i.e. the stability of the residue was improved when both flame retardants were combined. An improvement in flame retardancy for combinations of MPP with phosphorous compounds [45], as well as in combination with phosphate and boron compounds, has been reported for other systems as well [19,46]. It should be noted that the complex behaviour in LOI and UL 94 indicate quite different fire retardancy mechanisms, which had a specific influence on fire performance in different tests [42,47]. The ranking of time to ignition differed for various external heat fluxes in cone calorimeter investigations. For tests using 35 kW m2 irradiance, the ranking of ignition times was found to be PA66/GFeMPP < PA66/GF  PA66/GFeAlPi  PA66/ GFeAlPieMPP and no ignition for PA66/GFeAlPieMPPe ZnB; for 50 kW m2 it is PA66/GF ¼ PA66/GFeMPP < PA66/GFeAlPi ¼ PA66/GFeAlPieMPPeZnB < PA66/GFe AlPieMPP. The delayed ignition time of PA66/GF at low external heat fluxes was due to the formation of a thermooxidative surface layer [10]. This effect plays a minor role

for higher external heat fluxes. For PA66/GFeAlPieMPPe ZnB and PA66/GFeAlPieMPP the formation of an efficient surface layer was observed, which was efficient enough to avoid homogeneous ignition or even any ignition, especially when an external heat flux of 35 kW m2 was used. Using an external heat flux of 50 kW m2, the ranking of PA66/ GFeMPP, PA66/GFeAlPi, PA66/GFeAlPieMPPeZnB and PA66/GFeAlPieMPP correlated with the first main mass loss of TG investigations under air (Table 3). This may indicate that the barrier properties were less important then. 3.6. Fire behaviour: heat release The HRR and THR of cone calorimeter tests using external heat fluxes of 50 kW m2 are illustrated in Fig. 7 and the complete cone calorimeter results are summarized in Table 4. The comparison of THE and PHRR is illustrated in Fig. 8. The materials showed very different fire behaviour, especially with respect to the formation of a surface layer. Indeed, using an external heat flux of 35 kW m2, the protection by the surface layer resulted in unburned material remaining for all materials, at least under the frame edges. For PA66/GF the HRR was characterized by an intensive peak, followed by a shoulder, which is a characteristic HRR behaviour for glass-fibre materials [48]. The sharpness of the peak depended on the external heat flux. The THE of about 120 MJ m2 corresponds to values in the literature, taking

Fig. 7. HRR and THR versus time for all samples at an external heat flux of 50 kW m2 (solid line ¼ PA66/GF, open circles ¼ PA66/GFeMPP, open squares ¼ PA66/GFeAlPi, solid squares ¼ PA66/GFeAlPieMPP, solid rhombi ¼ PA66/GFeAlPieMPPeZnB).

U. Braun et al. / Polymer Degradation and Stability 92 (2007) 1528e1545

1539

Table 4 Cone calorimeter data Ex. heat flux

THE

PHRR

Residue

THE/ TML

MJ m2

kW m2

%

kW m2 g1

15

30

2

TSR e

TSR/TML

TCOR

TCOR/ TML

g1

g

gg1

500

0.3

PA66/GF

35 50 70

124 120 124

328 353 443

39.3 37.0 36.8

3.2 3.0 3.1

999 1493 1623

24 35 37

0.92 0.51 0.62

0.024 0.013 0.014

PA66/GFeMPP

35 50 70

84 116 96

150 246 260

49.9 37.1 40.9

2.9 2.9 2.6

1402 2293 2668

43 54 64

0.74 0.97 0.90

0.026 0.024 0.024

PA66/GFeAlPi

35 50 70

63 84 110

157 198 375

61.2 48.9 39.8

2.5 2.5 2.8

2086 3296 3264

63 84 78

1.68 2.50 3.67

0.066 0.075 0.093

PA66/GFeAlPieMPP

35 50 70

91 98 106

123 156 258

51.0 44.8 41.5

2.9 2.8 2.8

2551 2642 2180

73 71 54

1.86 2.40 2.69

0.059 0.069 0.072

PA66/GFeAlPieMPPeZnB

35 50 70

e 103 91

e 96 250

75.8 49.6 47.9

e 3.0 2.6

1994 2169 2235

e 61 58

0.42 2.71 1.88

e 0.080 0.054

Error based on maximal deviation of averaged values.

the changed glass-fibre contents and sample thickness into account [10,49]. When MPP was added to PA66/GF the PHRR was reduced by around 30e50% to values of 150e 260 MJ m2. The HRR process following the peak was marginal compared to PA66/GF and became a steadily and slightly decreasing HRR, typical for a residue-forming material. For THE only a slight reduction was observed. The presence of AlPi in PA66/GF reduced the heat release. The PHRR was about 160e370 MJ m2, which corresponded to a reduction of 15e50%. The THE was reduced compared to PA66/GF in the very similar order of 10e55%. The principle shape of the HRR curve was similar to PA66/GF. Hence, the changes in heat release indicated flame inhibition. The reduction depended strongly and systematically on the external heat

flux. With decreasing irradiation the flame retardancy effect increased. This flame inhibition effect increases with decreasing external heat flux and corresponds to the superior LOI performance of PA66/GFeAlPi. Flammability tests are fire scenarios with a low external heat flux. For PA66/GFe AlPieMPP and PA66/GFeAlPieMPPeZnB an efficient barrier layer at the surface was formed. It prevented the homogeneous ignition of the samples, especially when an external heat flux of 35 kW m2 was used. With PA66/GFeAlPie MPPeZnB the best cone calorimeter performance was obtained: since it hardly ignited at all for an irradiation of 35 kW m2, no large lighting flame was generated. Small flames were visible only in small cracks and holes of the surface. The mass loss, smoke and CO production of the 35 kW m2

Fig. 8. THE and PHRR versus external heat flux of all samples. Fits did not consider the 35 kW m2 measurements of the PA66/GFeAlPieMPPeZnB sample (open stars ¼ PA66/GF, open circles ¼ PA66/GFeMPP, open squares ¼ PA66/GFeAlPi, solid squares ¼ PA66/GFeAlPieMPP, solid rhombi ¼ PA66/GFeAlPie MPPeZnB).

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measurements may be due to thermo-oxidative decomposition. The HRR curves of PA66/GFeAlPieMPP and PA66/GFe AlPieMPPeZnB for lower irradiations (35 or 50 kW m2) were typical for a char-forming material. After an initial increase in HRR during the formation of the barrier layer, the HRR showed a steady smooth decrease. Using external heat fluxes of 50 or 70 kW m2, the reductions in PHRR for PA66/ GFeAlPieMPPeZnB (120e260 kW m2) and for PA66/ GFeAlPieMPPeZnB (100e250 kW m2) were similar and in the order of 40e70%. The THE (about 100 MJ m2) values were similar for both materials. For the analysis of the flame retardant mechanism the THE/ TML was determined (Table 3). A reduction of this value indicates flame inhibition or fuel dilution [4,11,28,50]. For PA66/GFeMPP the THE/TML showed a slight systematic reduction compared to PA66/GF. According to the literature [13,14,33] this is caused by fuel dilution due to the release of non-combustible pyrolysis products. For PA66/GFeAlPi the THE/TML was clearly reduced compared to PA66/GF and PA66/GFeMPP. Consequently the main fire retardancy mechanism of AlPi in PA66/GFeAlPi is flame inhibition. The gas-phase action of phosphorus decreased when high external heat fluxes were used. It increased when low external heat fluxes were used. So the gas phase action of AlPi was very efficient in the LOI test and yields a very impressive LOI value. For PA66/GFeAlPieMPP and PA66/GFeAlPieMPPe ZnB the THE/TML data were reduced only moderately compared to PA66/GF or PA66/GFeMPP. So the strong gas-phase action of AlPi in PA66/GFeAlPi is reduced in combination with MPP and MPP/zinc borate, respectively. Similar results have been reported for other materials [33,50]. The presence of a barrier reduces the effectiveness of phosphorus with respect to flame inhibition. 3.7. Fire behaviour: residue analysis The amount of residue measured using external heat fluxes of 50 or 70 kW m2 increases in an ascending order: PA66/ GF ¼ PA66/GFeMPP < PA66/GFeAlPieMPP < PA66/GFe AlPi < PA66/GFeAlPieMPPeZnB (Table 4, averaged values).

A different ranking results for the high temperature in TG investigations: PA66/GF < PA66/GFeMPP ¼ PA66/GFeAlPi < PA66/GFeAlPieMPP ¼ PA66/GFeAlPieMPPeZnB (Table 2). The difference is based on the absolute values of PA66/GFe AlPi and PA66/GFeAlPieMPPeZnB in TG (35 and 40 wt%) and cone calorimeter experiments (44 and 48 wt%). For both materials the pyrolysis did not occur completely during combustion in the cone calorimeter. The residues obtained in the cone calorimeter were homogenised and analysed by means of ATReFTIR. PA66/GF and PA66/GFeMPP showed similar spectra using different external heat fluxes. The spectra of the PA66/GF residue showed a broad band and a small band around 950 and 1400 cm1, respectively, which are attributed to glass fibres. PA66/GFeMPP also showed these glass-fibre signals and additional bands at 1075 and 950 cm1 as well as a broad band around 1600 cm1. Whereas the former were attributed to phosphorus esters and polyphosphates, the latter derived from polyaromatic char content. The infrared analysis of the materials containing phosphinate differs when different external heat flues are used (Fig. 9). At low external heat fluxes the PA66/GFeAlPi residue showed a broad shoulder around 1080 cm1. This shoulder increased to a sharp peak at 1115 cm1, when higher external heat fluxes were used. A similar shift in the absorption bands of aluminium phosphates were found in the TG residues. Both the absorption signal at 1115 cm1 and the additional band at 720 cm1 were attributed to aluminium phosphates, such as aluminium polyphosphate, aluminium pyrophosphate, or aluminium orthophosphate. The slight difference to TG results indicates that different mixtures of phosphates are formed. PA66/GFeAlPieMPP showed absorption bands of aluminium phosphates in all residues. For PA66/ GFeAlPieMPPeZnB aluminium phosphates were formed only after the combustion process with an external heat flux of 70 kW m2, whereas under moderate combustion conditions the main absorption bands shifted to 1075 cm1 and an additional small band at 720 cm1 was detected. It is concluded that the formation of aluminium phosphate in PA66/GFeAlPi, and PA66/GFeAlPieMPPeZnB depends on decomposition conditions. When the sample decomposes

Fig. 9. ATReFTIR of residues obtained in the cone calorimeter after combustion at 35, 50 and 70 kW m2 for (a) PA66/GFeAlPi, (b) PA66/GFeAlPieMPP and (c) PA66/GFeAlPieMPPeZnB; all compared with PA66/GF residue obtained in the cone calorimeter at 35 kW m2 (lower spectra).

U. Braun et al. / Polymer Degradation and Stability 92 (2007) 1528e1545

1541

Fig. 10. SEM pictures of the residue obtained in the cone calorimeter (irradiation ¼ 35 kW m2) of PA66/GFeAlPieMPPeZnB, surface layer (a) and middle of the sample (b).

very quickly, as happened in cone calorimeter experiments with high external heat fluxes, the formation of aluminium phosphates competes with the vaporisation of AlPi. Hence phosphorus action in the gas phase was decreased (see THE/ TML). When AlPi decomposes the resulting phosphinate ion can induce the carbon char formation of the polymer. As already shown for PA66/GFeAlPi and PA66/GFeAlPie MPPeZnB increased residue amounts were found. On the other hand, AlPi vaporisation was improved when lower external heat fluxes were used. This behaviour was verified with residue ATReFTIR measurements of PA66/GFeAlPi samples, heated with different heating rates. This result corresponded to the described LOI and THE/TML behaviour. The residue of PA66/GFeAlPieMPPeZnB from the cone calorimeter investigation at 35 kW m2 external heat flux was analysed without homogenisation with respect to morphology and chemical composition using SEMeEDX. On top of the sample a glassy surface was found, followed by a layer of hollow globules. The inside of the sample is dominated by large hollow ranges, stabilised only by glass fibres (Fig. 10a). From the middle of the sample through to its backside a dense, bulky material dominates the residue (Fig. 10b). It was identified by chemical analysis as non-decomposed polymer. The different areas of the residue were different not only in morphology, but also in chemical composition (Table 5). The surface is dominated by low carbon contents and high contents of heteroatoms (nitrogen, oxygen, phosphorus). Compared to the fire residue of PA66/GF, the heteroatom content was significantly higher in the residue of PA66/GFeAlPieMPPeZnB. This

indicated carbon char cross-linked with phosphate or amidophosphate structures. With increasing depth of sample an increase in carbon and a decrease in flame retardant components (phosphorus, aluminium) was observed. The chemical composition of the sample reached the composition of original PA66/GF material at the back side of polymer. This indicated that more or less original polymeric materials remained on the back side of the sample. So the SEMeEDX investigations showed that the residue composition is quite inhomogeneous and that this inhomogeneous composition causes the strong barrier effect. The surface layer, the hollow globules and the hollow areas result in a very effective protection against heat from the heat sources (cone heater, flame). The residue of the cone calorimeter investigation at 50 kW m2 external heat flux was analysed in detail by 11B, 31 P and 27Al NMR. Such NMR investigations have been used successfully to illuminate the interactions of phosphorous flame retardants with additives containing metal ions [51e53]. The material taken from the surface layer and from the bulk of the residue was analysed; both samples were homogenised. The 11B, 31P and 27Al NMR spectra are shown in Fig. 11 from top to bottom, respectively. In each graph the surface layer spectrum is plotted on top, whereas the spectrum of the ‘‘bulk’’ region is always shown underneath. As seen in the top part of Fig. 11, planar BO3 units dominate the borate structure in the solid residue (assigned to the resonance B1). Their NMR line shape is governed by second-order quadrupolar effects. These second-order effects scale inversely

Table 5 Chemical composition of the residues obtained in the cone calorimeter (irradiation ¼ 35 kW m2), based on SEMeEDX analyses, using an acceleration voltage of 9 or 16 eV In at.%

C

N

O

P

Al

Si

Surface residue PA66/GF (16 eV)

84

8

7

e

e

e

57 62 66 77 78

15 6 5 5 9

21 18 20 14 9

4 8 3 1 e

2 3 3 1 e

0.5 1.1 2.0 0.5 e

Top side

Surface (9 eV) Hollow balls (9 eV) Middle Matrix (16 eV) Back side Matrix (16 eV) Original PA66/GF, Au vaporised (16 eV)

PA66/GFeAlPieMPPeZnB PA66/GFeAlPieMPPeZnB PA66/GFeAlPieMPPeZnB PA66/GFeAlPieMPPeZnB

Averaged values, elements of less than 0.5 at.% are not considered.

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Finally, the 27Al spectra of surface layer and bulk consist of three signals assigned to A1, A2 and A3. The dominating signal A1 has an asymmetric shape and must be attributed to the glass fibres present in the sample (tetrahedrally coordinated AlO4 units in a silicate network). Further, a minor signal A2 is found on the right shoulder of this resonance at about 40 ppm. This chemical shift is typical for AlPO4 structure, where the tetrahedrally coordinated Al is coordinated with phosphate groups sharing the oxygen. A possible structural interpretation as amorphous AlO5 units in an amorphous aluminate network (no coordination to phosphate units) is excluded for various reaons: the expected peak position is shifted to somewhat lower field by several ppm, the 27Al line shape is expected to be broader and more asymmetrically broadened and the NMR spectrum does not show any hint on the required AlO6 and AlO4 groups of such a hypothetical aluminate network. Furthermore, the signal A3 with a peak position at 17 ppm is attributed to octahedrally coordinated AlO6, surrounded by phosphate groups. Both of these assignments A2 and A3 are in agreement with the 31P line shape. At higher temperatures (surface layer) the relative amount of octahedrally coordinated Al species decreases, and more AlO4 groups were formed, corresponding to the higher degree of condensation of the amorphous aluminium phosphate network. On the basis of NMR investigations it is proved unambiguously that aluminium and boron phosphates are formed, which acted as excellent barriers during combustion. A similar conclusion was drawn from experiments of boron oxide and ammonium polyphosphate in epoxy materials, here the borates provides a hard and mechanical resistant char [26,54]. Fig. 11. NMR of residues obtained in the cone calorimeter (irradiation ¼ 50 kW m2) of PA66/GFeAlPieMPPeZnB.

3.8. Fire behaviour: smoke and CO release

with the applied magnetic field strength. Hence under our experimental condition, these BO3 signals appear quite narrow (compared to measurements at lower field strengths), and e most importantly e they do not overlap with the signals of the tetrahedral BO4 units (indicated by B2 in Fig. 11). The 11B NMR spectra show two different BO4 groups. The left of the two lines (d ¼ 1 ppm) belongs to BO4 units of an amorphous borate network that also contains BO3 groups, whereas the right B2 signal can be attributed to BPO4 due to its chemical shift of about 4.5 ppm. Therefore 11B NMR verifies the presence of some BPO4, particularly in the surface layer where the highest temperatures were reached. The 31P NMR spectra (middle part of Fig. 11) support this conclusion. The main signal P1 points to an amorphous aluminium phosphate network, due to the broad resonance and its position at around 25 ppm. The peak position of the spectrum of the surface layer is shifted slightly to about 31 ppm, which corresponds to the BPO4 chemical shift and, thus, supports the conclusion drawn from the 11B spectrum. At lower temperatures (bulk region) two further resonances P2 and P3 are found at 17 ppm and 50 ppm. It is assumed that these lines arise due to direct PeC bonds from the remaining diethylphosphinate derivatives.

The release of incomplete combustion products such as smoke and CO was also detected and evaluated. On the one hand, they can provide a ranking in terms of fire hazards; on the other hand, they may support the analysis of fire retardancy mechanisms. A strong increase in smoke and CO quantity (by a factor of more than two) is caused by a suppressed total oxidation process and indicates a radical trapping mechanism. A decrease indicates a better ventilated combustion process, which dominates in the presence of a fuel diluter and barrier former [7,33]. As regards the TSR/TML and TCOR/TML it should be noted that these values also can be influenced by chemical processes in the pyrolysis and combustion zone. The increase of smoke release is ordered PA66/GF < PA66/ GFeAlPieMPPeZnB  PA66/GFeMPP < PA66/GFeAlPie MPP < PA66/GFeAlPi (TSR, TSR/TML in Table 4). The order of samples correlates with the flame-inhibition action of phosphorus: in PA66/GFeAlPi the phosphorus acts mainly in the gas phase, whereas the gas-phase activity of PA66/ GFeAlPieMPP and PA66/GFeAlPieMPPeZnB is reduced. In PA66/GFeMPP the phosphorus remains in residue. The increase of TCOR and TCOR/TML release is ordered PA66/GF < PA66/GFeMPP < PA66/GFeAlPieMPP < PA66/ GFeAlPieMPPeZnB < PA66/GFeAlPi. In CO production

U. Braun et al. / Polymer Degradation and Stability 92 (2007) 1528e1545

1543

Table 6 Influence of different fire retardancy mechanisms on the fire retarded materials, based on fire and pyrolysis behaviour investigations Main FR mechanism

PA66/GFeMPP

Flame inhibition

Fuel dilution Inorganic barrier formation

PA66/GFeAlPieMPP

PA66/GFeAlPieMPPeZnB

þþ Al[O(O)P(C2H5)2]3

þ HO(O)P(C2H5)2

þ HO(O)P(C2H5)2 Al[O(O)P(C2H5)2]3 þ Melamine þþþ Boron/aluminium phosphate þa

þ Melamine þ Polyphosphate

Carbon char formation a

PA66/GFeAlPi

þ Melamine þþ Aluminium phosphate þa

Only for high external heat fluxes.

the correlation of phosphorus flame inhibition action is also reflected. 3.9. Fire retardancy mechanisms The fire retardancy effects of all materials are summarized in Table 6. MPP in PA66/GF decomposed during pyrolysis. The melamine unit was released in the gas phase and acted by means of fuel dilution, whereas the polyphosphate acted as a barrier in the solid phase. Although the phosphorus influenced the decomposition of the polymer, no significant amount of stable char was created. The interaction observed is based on a Lewis acidebase reaction and was characterized by decreased thermal stability, separation of the decomposition process and changes in the cyclopentanone and carbon dioxide release. However, the polyphosphate content in PA66/ GF is too low for an effective protection against flames. The AlPi additive in PA66/GF was mainly vaporised prior to decomposition due to its high thermal stability. It acted in the gas phase as a flame inhibitor. As for the melamine polyphosphate, an interaction occurred with the polymer decomposition. According to its lower oxidation state, and consequently weaker Lewis base characteristic, the interaction with the polymer matrix was only moderate. The vaporisation of AlPi decreased with increasing external heat flux and aluminium phosphate was formed instead. Hence, the lower the external heat flux during combustion (LOI) was, the larger the flame retardant effect obtained. On the other hand, because the residue has no satisfactory stability, the materials failed to achieve a V-0 classification in the UL 94. The combination of AlPi and MPP in PA66/GF resulted in the formation of aluminium phosphate, because the strongest Lewis base (phosphate) reacted with the strongest Lewis acid (aluminium). The aluminium phosphate acted as a very effective barrier. These phosphates stabilised the residue and in combination with the reduced gas phase action of phosphorus the material achieved a V-0 classification in the UL 94 test. In the presence of zinc borate the reactivity of AlPi and MPP changed, because boron is a stronger Lewis base than aluminium. Boron phosphate was formed and only the hyperstoichiometric content of phosphate reacted with aluminium, forming aluminium phosphate. The boronealuminium phosphate barrier formed was more effective than aluminium phosphate. The excellent barrier characteristics were featured in an

inhomogeneous complex residue. The aluminiumeboron phosphates gave the material the stability for UL 94 test as the aluminium phosphates did in PA66/GFeAlPieMPP. In the materials PA66/GFeAlPi, PA66/GFeAlPieMPP and PA66/GFeAlPieMPP, the formation of char and the subsequent removal of polymeric material from the pyrolysis process was not a dominant flame retardancy effect, as is known for PA66/GF with red phosphorus [10] and PA66/GF with high amounts of MPP [13]. 4. Conclusion The pyrolysis and fire retardant behaviour of PA66/GF containing AlPi, MPP, AlPi þ MPP, and AlPi þ MPP þ zinc borate were studied in order to investigate the dominant fire retardancy mechanisms in these glass-fibre reinforced materials. Models for the main decomposition pathways are proposed based on a comprehensive characterization of thermal behaviour (TG) and the volatile and non-volatile decomposition products (TGeFTIR, FTIReATR). The flammability of the materials was investigated using LOI and UL 94, and the fire behaviour for forced-flaming conditions using a cone calorimeter at different irradiations. Fire residues were analysed by means of FTIR, SEMeEDX and NMR investigations. The flame retardancy mechanisms were discussed for the gas and condensed phases. The results not only explain the fire-retardancy effects in the different materials, but also illuminate principles in the interaction of PA66/GF with phosphorous containing additives. The addition of MPP to PA66/GF as a flame retardant is based on a fuel-dilution effect and a phosphate barrier. Most AlPi vaporises and acts very effectively as a flame inhibitor in the gas phase. Combining both additives alters the flame retardancy mechanisms. Now the effective barrier layer of aluminium phosphate dominates the fire protection; the flame inhibition of vaporised phosphinic acid and fuel dilution of vaporised melamine decomposition products became less important. The combination of MPP, AlPi and zinc borate further improves barrier formation, because the boronealuminium phosphate layer formed shows better barrier performance than the aluminium phosphate layer. The reactivity of the various phosphorus species is caused by Lewis acidebase interactions. The work shows that the reactivity of the phosphorus additive with the polymer matrix influences not only the activity of phosphorus in the gas or condensed phase, but also the interaction

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