Study of the thermal degradation of an aluminium phosphinate–aluminium trihydrate combination

Thermochimica Acta 551 (2013) 175–183

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Study of the thermal degradation of an aluminium phosphinate–aluminium trihydrate combination Sophie Duquesne a,b,c,∗ , Gaëlle Fontaine a,b,c , Oriane Cérin-Delaval a,b,c , Bastien Gardelle a,b,c , Grégory Tricot a,d,e , Serge Bourbigot a,b,c a

Univ Lille Nord de France, F-5900 Lille, France ENSCL, ISP-UMET, F-59652 Villeneuve d’Ascq, France c CNRS, UMR 8207, F-59652 Villeneuve d’Ascq, France d USTL, UCCS, F-59652 Villeneuve d’Ascq, France e UMR CNRS 8181, F-59652 Villeneuve d’Ascq, France b

a r t i c l e

i n f o

Article history: Received 14 August 2012 Received in revised form 25 October 2012 Accepted 28 October 2012 Available online 16 November 2012 Keywords: Thermal decomposition Diethyl aluminium phosphinate Aluminium trihydroxide Solid state NMR

a b s t r a c t The thermal degradation of aluminium diethylphosphinate (AlPi) with aluminium trihydrate (ATH), two flame retardant additives, was investigated. The interactions between the additives were characterized using thermal analysis (TG). The evolved gas were analysed by TGA–FTIR and the degradation products formed in the condensed phase were fully characterized using solid state NMR analysis, and in particular 2D NMR, as well as XRD analyses. Decomposition pathways have been determined and it was proposed that chemisorption of the phosphinate on alumina resulting from the ATH dehydration, activated by the temperature increase, influences the degradation of the mixture. As a consequence, the degradation products of AlPi differ in presence of ATH since the formation of aluminophosphonate is prevented. © 2012 Elsevier B.V. All rights reserved.

1. Introduction Polymers are generally organic based material and due to their composition are highly flammable. In order to improve their flammability properties, flame retardant additives are generally added into the polymeric matrices. These additives can be used at relatively large amount depending on their nature. As an example, aluminium trihydrate or magnesium dihydrate is added at content around 50–60 wt.% to achieve a satisfactory degree of flame retardant properties. In order to decrease the flame retardant additives content, while maintaining the properties, combination of additives are usually used to develop synergism. The flame retardant synergisms provided by those combinations can be generally explained by chemical or physical interactions between the additives. Synergism is reported between numbers of additives. As aluminium phosphinate (AlPi) is concerned, its combination with melamine cyanurate [1] and melamine polyphosphate [2] is reported and results in the modification of the mode of action of the AlPi from a gas phase mode of action to a charring and dominating barrier effect and thus to a synergistic effect. This effect was

∗ Corresponding author at: Ecole Nationale Supérieure de Chimie de Lille, UMETISP, Cité Scientifique Avenue Mendeleïev CS 9010859652 Villeneuve d’Ascq. Tel.: +33 0320 33 72 36; fax: +33 0320 43 65 84. E-mail address: [email protected] (S. Duquesne). 0040-6031/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.tca.2012.10.025

reinforced in presence of zinc borate [2]. It was also shown that a synergistic effect could be obtained combining AlPi and organomodified montmorillonite (o-MMT) [3]. The mode of action was attributed to a physical effect, the presence of o-MMT modifying the charred barrier properties. Metal oxides also show synergistic effect with AlPi promoting the formation of stable carbonaceous structure acting as a protective barrier [4]. Moreover, redox reaction between iron oxide and AlPi was also evidenced [5]. On the other hand, synergism of metal hydroxide (MeH) (aluminium trihydroxide (ATH) or magnesium dihydroxide (MDH)) with co-additives (zinc borate [6,7], phosphorus-based additives [8,9], nanoparticules [10–13], silicon-based additives [14–16], etc.) is widely reported in the literature. In the case of MeH/ZB combination, the synergism was attributed to the formation of a vitreous protective coating that leads to a more efficient protective barrier. In the case of ammonium polyphosphate based system, it was revealed that the formation of magnesium phosphate stabilizes the phosphorus in the system and thus leads to a synergistic effect. Recently, it was reported that the combination of AlPi and ATH provides outstanding flame retardancy performances in EVA [17]. As an example, the Limiting Oxygen Indexes (LOIs) of materials containing 190 phr of additives were reported. Surprisingly, whereas the material containing only ATH present a LOI of 51 vol.%; the one combining ATH and AlPi at 160/30 phr ratio and 100/90 phr ratio present an increase in the LOI respectively of 6 and 8 vol.%. The objective of this study is thus to study what kind of interaction

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occurring between this two compounds enable explaining the good flame retardant properties observed when both AlPi and ATH are combined in EVA. In this scope, the thermal degradation mechanism of the ATH–AlPi combination will be determined. The degradation mechanism will be elucidated analysing the degradation products released in the gas phase as well as those formed in the condensed phase using adapted techniques (thermogravimetric analyses (TGA) coupled with infrared spectroscopy, solid state NMR and X-ray diffraction analyses). 2. Experimental 2.1. Materials The materials used in this study are diethyl aluminium phosphinate (Exolit AlPi supplied by Clariant) and aluminium trihydroxide (Al(OH)3 , Apyral 120E supplied by Nabaltec). Both additives were used as received. A combination of the two additives at a ratio 1/1 (corresponding to a Al/P ratio of 2) was prepared mixing two equivalent mass of additives. This ratio was chosen first because it is in the range of ratio used for materials demonstrating improved fire behaviour as previously reported in the literature [17]. Moreover, the high quantity of phosphorus and aluminium enables achieving accurate solid state NMR analysis. 2.2. Thermal analysis Thermogravimetric (TG) experiments were performed using a TGA Q5000 (TA Instruments) with a balance purge flow of 10 ml min−1 and a sample purge flow (nitrogen) of 50 ml min−1 . The samples (15 ± 0.2 mg) were heated in 100 ␮l alumina crucibles from 50 ◦ C to 800 ◦ C at a heating ramp of 10 ◦ C/min. In order to determine potential interactions between ATH and AlPi when mixed together during their thermal degradation, difference weight loss curve (M) was calculated (Eq. (1)). It represents the difference between an experimental TG curve obtained on the mixture (Mexp ) and a calculated TG curve (Mcalc ) obtained from Eq. (2). M = Mexp − Mcalc

(1)

Mcalc = 0.5 × MATH + 0.5MAlPi

(2)

with MATH and MAlPi , the TG of respectively pure ATH and pure AlPi. The TG was coupled with a FTIR Nicolet is10 Spectrometer (ThermoFischer). A transfer line with an inner diameter of 1 mm was used to connect the TGA and the infrared cell. Both the transfer line and the gas cell were heated to 225 ◦ C to avoid the condensation of the gaseous degradation products. The IR spectra were recorded in the 400–4000 cm−1 spectral range. The spectra correspond to the accumulation of 8 scans with an optical resolution of 4 cm−1 . 2.3. Thermal treatments Residues obtained after different heat treatments were collected to investigate the condensed phase. Heat treatment consists to expose a sample at a determined temperature under controlled nitrogen flow using a tubular furnace. The treatment temperatures were determined according to TG curves and correspond to the characteristic degradation steps of the systems. The chosen experimental conditions were similar to that of the TG measurements: a heating rate of 10 ◦ C/min, from the ambient to the treatment temperature followed by an isotherm (15 min). The sample was then cooled to ambient temperature prior being analysed. Table 1 compares the weight loss observed after a heat treatment at a characteristic temperature with the expected weight loss obtained from TGA. It can be observed that the value are slightly

Table 1 Comparison of the weight loss obtained after heat treatment and TGA. Temperature (◦ C)

250

350

450

550

Weight loss after heat treatment (wt.%) Weight loss on TGA (wt.%)

9.5 7

12 17

27 35

58.5 61

different which is not surprising since the size of the sample is different leading to heat and mass transfer effect. However, it can be considered that if the weight loss obtained after the treatment in the furnace is in the range of the selected degradation step the heat treatment is appropriate for our approach (for example the second step of degradation of the mixture ATH/AlPi leads to a mass loss between 18 and 61 wt.%, thus the weight loss in the furnace has to be comprised within these values). 2.4. Solid state NMR Residue analysis was performed using solid state Nuclear Magnetic Resonance (NMR). 27 Al NMR measurements were carried out on a Bruker Avance II 400 (B0 = 9.4 T) with a probe head of 4 mm using magic angle spinning (MAS = 13 kHz). The repetition time was fixed at 1 s for all samples, a minimum of 1024 scans was necessary to obtain a satisfactory signal to noise ratio. Al in aqueous solution was used as a reference. To increase the resolution of some spectra and to reduce induced quadrupolar effects, some measurements were realized on a Bruker Avance II 800 with a probe head of 3.2 mm using MAS. The same conditions (rotation frequency and references) were used. 31 P NMR measurements were performed on a Bruker 400 spectrometer at a spinning rate of 13 kHz. Brucker probe heads equipped with 4 mm MAS assembly were used. The 31 P acquisitions were performed under 1 H decoupling conditions in order to improve the spectrum resolution. A repetition time of 120 s was applied for all samples. H3 PO4 in aqueous solution (85%) was used as a reference. The spatial proximity between the phosphorous and aluminium sites was investigated with the 2D MAS-NMR 27 Al{31 P} D-HMQC (Dipolar Heteronuclear Multiple Quantum Coherence) sequence [18]. This NMR experiment uses the standard HMQC pulse sequence to produce heteronuclear coherences but an additional pulse scheme allows for the creation of through-space coherences in place of the standard through bonds signals obtained with the basic HMQC sequence. As a consequence, the 2D spectrum displays correlation signals between spatially close atoms. The 27 Al{31 P} D-HMQC spectrum has been obtained at 9.4 T on a Bruker spectrometer equipped with a 4 mm triple channels probe operating at a spinning frequency of 12.5 kHz. The through space correlations were created by a scheme pulse based on the SFAM (Simultaneous Frequency and Amplitude Modulation) technique [19] applied during 1 ms. The 2D spectrum was recorded with the following acquisition parameters: number of scans = 640, repetition time = 0.5 s, number of points = 2474 × 120. 2.5. XRD analysis The measurements were carried out on a Brüker D8 equipped with a high temperature chamber XRK 900 and a Lynxeye detector using Cu K␣ radiation source. The experiments were performed under nitrogen flow, the powder samples were placed on a gold foil to avoid potential reaction with the alumina sample holder. From the room temperature to 800 ◦ C the sample were heated at a heating ramp of 0.05 ◦ C/min. For each 25 ◦ C increase, the temperature is stabilized and a DRX scanning is realized in the 2 range between 7◦ and 70◦ (with steps of 0.0195◦ ).

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0.8

3.5

100

3

0.6 0.5

80

0.4 0.3

70

0.2

residual mass (wt-%)

90

deriv. weight (wt-%/°C)

residual weight (wt-%)

0.7

(a)

(b)

80

2.5 2

60 1.5 40

1 0.5

der. weight (wt-%/°C)

100

177

20 0.1 60 0

100

200

300

400

500

600

0 700

Temperature (°C)

0 0 0

100

200

300

400

500

600

-0.5 700

Temperature (°C)

Fig. 1. TG (plain lines) and DTG (dotted lines) curves of (a) ATH and (b) AlPi (10 ◦ C/min in nitrogen).

3. Results and discussion 3.1. Thermogravimetric analysis The TGA analyses of the pure additives are presented in Fig. 1. As shown in Fig. 1a, the thermal degradation of aluminium trihydroxide (ATH 120E) occurs in a single step between 200 and 320 ◦ C with a maximum degradation rate at around 280 ◦ C. The final residue of 65 wt.% correspond to the formation of alumina (Al2 O3 ; M = 101.6 g/mol) resulting from the dehydration of ATH (M = 78 g/mol). The degradation of AlPi (Fig. 1b) also occurs in a single step from 390 to 520 ◦ C with a maximum degradation rate at 475 ◦ C. The pyrolysis mechanism of AlPi was already reported [3]. The AlPi first degrades leading to the formation of some carbonaceous char and aluminophosphonate Al2 (CH3 CH2 P(O)(O− )2 )3 composed of octahedral aluminium and phosphonate groups. The phosphorus compound then decomposes in a transitory aluminium phosphonate (penta-coordinated aluminium) which turns then into an amorphous aluminophosphate AlPO4 . During the decomposition of AlPi, it was demonstrated that AlPi could partially sublimate and/or that ethene and diethylphosphinic acid are released [1]. At 700 ◦ C, the final residue of 6.2 wt.% is composed of condensed carbonaceous structures coming from the condensation of the products resulting from the degradation of the diethyl part of AlPi and crystallized aluminophosphates. Fig. 2 compares the experimental TG curve and the calculated TG curve of the mixture ATH/AlPi and also presents the resulting difference weight loss curves (M). This curve allows pointing out a potential increase or decrease in thermal stability of the mixture.

Fig. 2. Experimental and calculated TG curves as well as difference weight loss curves of the ATH–AlPi combination (10 ◦ C/min, in nitrogen).

It appears that the combination of the two additives degrades into two steps occurring respectively in the temperature range of the degradation of ATH first and then of the one of the phosphinate at higher temperature. It is noteworthy that the experimental degradation curve and the calculated one are very close to each other except for the second step which is observed at lower temperature than expected (experimental curve is lower than calculated one for a given temperature). As a consequence, the difference weight loss curve shows a significant thermal destabilization (−13%) in the 400–500 ◦ C temperature range. It suggests that interaction between the additives and/or their degradation products should occur during the thermal degradation of AlPi. On the other hand, it is observed that the weight loss observed during the first degradation step is equal to 17 wt.%. This weight loss corresponds to the expected weight loss of water resulting from ATH dehydration (it was previously shown that the TGA of ATH show a mass loss of 35 wt.% within the same temperature range). The difference (34 wt.% versus 35 wt.%) is comprised in the experimental error. In order to understand and explain this observed phenomenon, the degradation products are then analysed using adapted technics. 3.2. Gas phase analysis The gaseous products of degradation were analysed using TGA–FTIR technique. The FTIR spectra versus time of the gases evolved during the TGA of the mixture ATH/AlPi carried out in nitrogen are presented in Fig. 3. The degradation begins at t = 25 min (corresponding to a temperature of 250 ◦ C) and ends at t = 75 min (corresponding to a temperature of 750 ◦ C). The two degradation

Fig. 3. FTIR spectra of the gases evolved during the pyrolysis of ATH–AlPi combination versus time.

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c

b

a

4000

3500

3000

2500

2000

1500

1000

Wavenumber (cm-1) Fig. 4. FTIR spectra of the gases evolved during the pyrolysis of ATH–AlPi combination at characteristic temperatures (a, 260 ◦ C; b, 450 ◦ C; c, 600 ◦ C).

steps are also clearly observed on this Figure. Indeed, it appears that some gases are detected first between 25 and 45 min and then between 65 min up to the end of the experiment. Spectra collected at characteristic times (or temperature) are then presented in Fig. 4. At the beginning of the pyrolysis (260 ◦ C) large bands between 3400–3600 cm−1 and 1300–1600 cm−1 assigned to the presence of water resulting from the ATH dehydration are observed. No other peaks are observed confirming that the first step of the degradation only corresponds to the ATH dehydration. At 450 ◦ C, some peaks are observed at 1154 and 1085 cm−1 . It was previously reported that those bands could be assigned to PO2 − anion characterizing the sublimation of the AlPi [2]. At 600 ◦ C, the spectrum shows an increase of the intensity of the bands linked to the phosphorous species previously reported for the second degradation step. Additional peaks are observed at 1274; 1245 and 850 cm−1 and could be attributed respectively to P O; P C and P O bonds suggesting the formation of diethylphosphinic acid. The presence of a band at 950 cm−1 is attributed to the release of ethene. It thus demonstrates that the P C bonds of the AlPi break, leading to the evolution of carbonaceous volatiles. This is consistent with the decomposition process of AlPi, since the degradation of aluminium diethyl phosphinate into aluminophosphates necessarily provokes the degradation of the ethyl groups [3]. Fig. 5 reports the evolved gases release rate versus time occurring during ATH/AlPi degradation for water, ethene, AlPi and diethylphosphinic acid. Those data were obtained following the area of the peak corresponding to the characteristic FTIR peaks of the different components versus time. It confirms that the first step of degradation corresponds to the dehydration of ATH since water is mainly released during this first step. On the other hand, during the second step of degradation, it is observed that both sublimation and degradation of AlPi leading to the formation of diethylphosphinic acid and ethene occurs. Moreover, it can be noted that the release of ethene is concomitant with the one of the phosphinic acids as it was expected. Finally, AlPi sublimation is observed up to the end of

the experiment. This could be explained by the fact that AlPi could condense in the transfer line. As a conclusion, the analysis of the gaseous products evolved when the mixture ATH/AlPi degrades show that both additives degrade independently. Indeed, the observed products, in agreement with the literature, are similar to those obtained when the additives degrades as single component. Those results suggest that either the interaction between the additives mainly occurs in the condensed phase or that this technique is not sensitive enough to detect some differences. In order to demonstrate if the first assumption is validated, analysis of the condensed phase is then carried out using X-ray diffraction analyses and solid state NMR spectroscopy. 3.3. Condensed phase analysis First, the X-ray diffractograms of the ATH–AlPi composition versus temperature are presented in Fig. 6. It appears that the non-heated material exhibits a crystalline structure, composed of gibbsite (ATH) (18◦ , 20.5◦ , 27◦ , 28◦ , 36.5◦ , 37.5◦ ) and crystallized AlPi (9.5, 19.5◦ , 31.5◦ , 34◦ ). The bands at 38, 44 and 65◦ correspond to the sample holder. There is no modification of the diffractogram up to 175 ◦ C, temperature at which the peaks corresponding to gibbsite structure decrease while the intensity of some bands linked to the AlPi crystals increases. Beyond 400 ◦ C, it does not remain any signal, except one from the holder (38◦ ), indicating that the heated material has become totally amorphous. Thus, the disappearance of the bands related to the phosphinate suggests its degradation. On the other hand, the crystalline aluminophosphate expected from the thermal degradation of AlPi [1–3] is not formed when AlPi is degraded in presence of ATH. It is thus suspected that during the dehydration of ATH some interaction occurs between the metal hydroxide and the phosphinate leading to a modification of the degradation pathway of the phosphinate. In order to investigate such interaction, the phosphinate–metal hydroxide combination was heat treated at 250; 350; 450 and 550 ◦ C. Those temperatures correspond to characteristic

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179

Extinction 0,06

Extinction 0,70

0,05

0,60

DiethylPhosphinic Acid 0,04

0,50

0,03

0,40

0,02

AlPi

H2 O

0,30

C2 H

0,01

0,20

0

0,10

-0,01

0,00 10

20

30

40

50

60

70

80

90

-0,02

-0,10 Time [min]

Fig. 5. Evolved gases release rate during ATH/AlPi degradation.

Fig. 6. High-temperature XRD of the ATH–AlPi combination (from 50 to 800 ◦ C, a diffractogram is collected every 25 ◦ C) (→: AlPi;

temperature of the degradation of ATH/AlPi mixture; the two first temperatures corresponding to the first degradation step (degradation of ATH) and the second ones to the second degradation step (degradation of AlPi). The residues of those thermal treatments were then analysed through 27 Al and dipolar-decoupling MAS 31 P solid state NMR. The spectra obtained through 27 Al NMR are reported in Fig. 7. The spectrum of the untreated sample presents three peaks at respectively −12, −3.2 and 7.9 ppm. The peaks at 7.9 and −3.2 ppm are attributed to the single octahedral resonance of aluminium in a gibbsite structure [20] and to the resonance of extra-framework aluminium in octahedral coordination [21]. The signal at −12 ppm, characterizes cationic Al3+ in octahedral coordination with phosphorus atoms in their second coordination sphere [19] and is thus

: ATH;

: sample holder).

assigned to the AlPi. When heated at 350 ◦ C, an additional peak appears around 65 ppm as a consequence of the elimination of structural water in alumina trihydroxide with the formation of a close-packed oxygen lattice in which some of the tetrahedral interstices are occupied by Al [22]. It is noteworthy that the signals linked to dehydrated ATH are very broad: this is consistent with the amorphization of the material. During the second degradation step (550 ◦ C), the peak at −12 ppm disappears, indicating that the degradation of the phosphinate compound has occurred. The two broad bands at 7.9 and 67 ppm assigned to Al2 O3 are still observed. Surprisingly, aluminophosphate was not detected. So, to ensure that the quadrupolar broadening of these signals (characterized by an asymmetric widening) does not hide any additional band, the same NMR analysis was performed at higher magnetic field (18.8 T

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Fig. 7. 27 Al MAS NMR spectra of the thermal degradation residues of the ATH–AlPi combination (degradation in nitrogen) at various heat treatment temperatures.

versus 9.4 T) to reduce induced quadrupolar effects and to increase the peak resolution. The spectrum (Fig. 8) reveals an additional band at 35 ppm. This signal is characteristic from penta-coordinated aluminium, in particular from AlO4 entities in an aluminophosphate structure. These AlO4 unities are generally observed at higher chemical shifts, but they are shielded because of the connectivity with phosphorous (shift towards the lower fields) [23]. To confirm the presence of this third signal the simulation of the spectrum was done using Dmfit software [24]. The resulting calculations confirm that there are three aluminium sites at 65, 35 and 3.7 ppm. It is noteworthy that the expected signals at −6 and −20 ppm (octahedral alumina (AlO6 ) connected by Al O P bridges) characteristic from aluminophosphonate [3] generated by the degradation of the pure phosphinate are not present on the spectrum. It is thus possible to conclude that the degradation products of AlPi are different when mixed with ATH. This hypothesis is then investigated by the analysis of the phosphorous species. The 31 P spectra of the ATH–AlPi heat treated at characteristic temperatures are presented in Fig. 9. From ambient to 450 ◦ C, the spectra exhibit two narrow peaks at 41.3 and 43.0 ppm. Those peaks correspond to phosphorus sites in phosphinate compounds (CH3 CH2 P(O) O− ) [26]. They are detected at higher chemical shifts than expected because of the deshielding effect induced by the connectivity of the phosphinate with the Al3+ cation [27]. It is noticeable that the intensity of the two peaks varies between 250 and 450 ◦ C (the respective intensities are inversed). These two

Fig. 8. 27 Al MAS NMR spectra (800 MHz) of the thermal degradation residue of the ATH–AlPi collected after a heat treatment carried out at 550 ◦ C (experimental (blue) and simulated (red) spectrum). The spectrum has been simulated using the extended Czjcek model [25] with three components characterized by the following chemical shift/quadrupolar constant values: 69 ppm/4.1 MHz, 46 ppm/3.3 MHz and 11 ppm/4.6 MHz. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of the article.)

Fig. 9. 31 P DD-MAS NMR spectra of the thermal degradation residues of the ATH–AlPi combination (degradation in nitrogen) for different heat treatment temperature; *spinning sideband.

bands are assumed to correspond to two crystalline forms of AlPi and thus modification of the crystalline structure of AlPi between 20 and 350 ◦ C should occur. For a heat treatment carried out at 550 ◦ C, the two peaks at 41.3 and 43.0 ppm disappear whereas two broad bands centred on 17.0 ppm (very low intensity) and −15.5 ppm appear. It thus demonstrated that the degradation of the phosphorous compound begins after 450 ◦ C. The band at 17 ppm is attributed to phosphonate structures [29]. Moreover, since the aluminium signals do not exhibit the peaks of aluminophosphonate (−6 and −20 ppm), the hypothesis of the formation of diethyl phosphonic acid seems more reasonable. The broad signal at −15.5 ppm could be the overlap of several peaks and thus deconvolution of the spectrum was performed using Dmfit. The simulation (Fig. 10) involves the presence of four peaks at −10.9, −18.7, −25 and −30 ppm to obtain a good agreement between calculated and experimental signals. Those signals between −10 and −30 ppm are characteristic of tetrahedral phosphates connected to aluminium centres [28]. Pyrophosphates can also be evoked, since they produce a signal around −10 ppm [30]. The broadening of the peak

Fig. 10. Experimental 31 P spectrum (in blue) and simulated one (in red) of ATH/AlPi mixture heat treated at 550 ◦ C. The simulation has been obtained with a 5 components system using lorentzian line shape signals characterized by the following chemical shift/full width at half-maximum values: 19.2 ppm/13.8 ppm, −10.2 ppm/11.9 ppm, −18.5 ppm/8.3 ppm, −24.6 ppm/8.7 ppm and −30 ppm/0.9 ppm. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of the article.)

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181

Fig. 12. 1 H MAS-NMR spectrum of the ATH–AlPi mixture heat treated at 550 ◦ C (800 MHz).

Fig. 11. 2D MAS-NMR 27Al(31P)D-HMQC spectrum of the ATH–AlPi mixture after heat treatment carried out at 550 ◦ C under nitrogen.

at −15,5 ppm thus results from the large distribution of different phosphorous species in a glassy structure. Those different characterizations show that the degradation of the phosphinate–ATH mixture leads to the formation of an amorphous residue containing alumina; aluminium phosphates and phosphonic acid. Since the signal between −10 and −30 ppm on the 31 P spectrum was very broad it was not possible to clearly identified the different phosphate-based structure present in the

residue. 2D MAS-NMR D-HMQC sequence and 1 H MAS experiment were thus performed on the collected residue of the ATH/AlPi after a heat treatment at 550 ◦ C in order to obtain information on the spatially close atoms. The spatial correlation between phosphorus and aluminium nuclei (Fig. 11) reveals that the aluminium signals (tetra, penta and octahedral sites) are corellated to the phosphorous signals located at −10 and −30 ppm. The correlation signals between octahedral aluminium sites (10 ppm) and these phosphorous sites are particularly intense, suggesting that this signal is linked to aluminophosphates. In that structure, the phosphates are composed of tetrahedral phosphorus surronded by four aluminium second nearest neighbour (P(OAl)4 sites) [23]. It is interesting to note that the 31 P signals betweens −10 and −30 ppm also exhibits correlation with tetrahedral 27 Al peaks (peak at around 65 ppm). This indicates the co-presence of AlO4 and AlO6 species. There must be at least two Al atomes in the phosphorus second coordination sphere, one tetrahedral and one octahedral aluminium [31]. All aluminium sites are connected to phosphorous nuclei, but the phosphorous sites generating the signal at 17 ppm on the 31 P spectrum (previously attributed to phosphonate structures) have no aluminium in

Fig. 13. Comparison of the thermal degradation scheme of the pure (a) AlPi and (b) ATH–AlPi combination in inert conditions.

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Fig. 14. Comparison of the picture of the collected residue after a heat treatment at 450 ◦ C of (a) pure AlPi and (b) mixture of ATH/AlPi.

Fig. 15. Chemical binding of phosphinate during the thermal degradation.

their neighbourhood. Such result confirms that this signal does not correspond to aluminium phosphonates. It so validates the assumption that the band at 17 ppm could be attributed to phosphonic acid [32]. 1 H MAS experiment (Fig. 12) demonstrates that the material still contains protons: the bands between 1 and 3 ppm can be related to proton belonging to alkyl compounds while the broad signal at 5.5 ppm can be attributed to P OH groups [31]. Such a signal is compatible with the presence of organic phosphonic acid. As a conclusion, it has been demonstrated that the ATH–AlPi mixture degrades into aluminophosphates (phosphorus sites with AlO6 and AlO4 ) and some organic phosphonic acid (most probably ethyl phosphonic acid). 3.4. Degradation scheme The previously described results allow drawing a global degradation scheme, depicted in Fig. 13. Aluminium trihydrate first degrades into alumina evolving water. Then, the mixture of amorphous alumina and aluminium diethyl phosphinate decomposes to form amorphous aluminophosphates and ethyl phosphonic acid in the condensed phase. Fig. 13 also compares the degradation pathway of pure AlPi with that of the mixture ATH/AlPi. The main differences between the degradation of pure AlPi and of the mixture can thus be drawn. It is observed that the formation of aluminophosphonate, expected when AlPi degrades, did not occur in presence of ATH where ethyl phosphonic acid was obtained in the condensed phase. Moreover, there is no charring of AlPi occurring in presence of ATH. This can also clearly been observed comparing the corresponding collected residues after a heat treatment (Fig. 14). Indeed, whereas a black and expanded residue is obtained in the case of pure AlPi, a white powder was observed for the mixture ATH/AlPi. This differed from the degradation of the pure phosphinate [3]. As a consequence, the mode of action of AlPi in presence of ATH should be preferentially

a gaz phase mechanism, whereas both gaz and condensed phase action are generally reported. It could thus be assumed that interaction between the two additives assigned to the chemisorption of the phosphinate, activated by the temperature increase, on alumina occurs (Fig. 15). The phosphinate is linked to gibbsite at first by hydrogen bonds. Then, the dehydration of gibbsite leads to the chemisorption of the phosphorous compound by oxygen bonding. This chemisorption, modifying the crystallinity of the phosphinate, is consistent with the NMR analyses revealing a modification of the AlPi structure versus temperature. At the final stage aluminophosphates and ethyl phosphonic acid are formed. According to the literature, the proposed mechanism is compatible with the temperatures of the degradation [33]. 4. Conclusion In this study, the mechanism of degradation of ATH/AlPi mixture was investigated. The condensed phase was characterized using Xray diffraction and solid state NMR. It was concluded that a chemical interaction between ATH and AlPi occurs leading to a modification of the mechanism of degradation of AlPi in the condensed phase. Chemisorption of the phosphinate on alumina results in the formation of diethyl phosphonic acid and aluminophosphate with different coordination sphere. Acknowledgement Bertrand Revel is kindly acknowledged for its skilful technical assistance and his expertise in NMR. References [1] U. Braun, B. Schartel, Flame retardancy mechanisms of AlPi in combination with melamine cyanurate in glass-fibre-reinforced poly(1,4-butylene terephthalate), Macromol. Mater. Eng. 293 (2008) 206–217.

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