(APP/MPP/TiO2) in PMMA

Polymer Degradation and Stability 97 (2012) 2154e2161

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Investigation of fire-resistance mechanisms of the ternary system (APP/MPP/TiO2) in PMMA Blandine Friederich a, b, Abdelghani Laachachi a, *, Michel Ferriol b, Marianne Cochez b, Rodolphe Sonnier c, Valérie Toniazzo a, David Ruch a a b c

Department of Advanced Materials and Structures (AMS), Centre de Recherche Public Henri Tudor, 66 rue de Luxembourg, BP 144, L-4002 Esch-sur-Alzette, Luxembourg Université de Lorraine, Laboratoire Matériaux Optiques, Photonique et Systèmes, E.A. 4423, Metz F-57070, France Centre des Matériaux de Grande Diffusion (CMGD), Ecole des Mines d’Alès, 6 avenue de Clavières, F-30319 Alès, France

a r t i c l e i n f o

a b s t r a c t

Article history: Received 16 March 2012 Received in revised form 17 July 2012 Accepted 20 August 2012 Available online 6 September 2012

The thermal and fire-resistance properties of the ternary system ammonium polyphosphate/melamine polyphosphate/titanium dioxide (APP/MPP/TiO2) was studied in poly(methyl methacrylate) (PMMA). PMMA-7.5%APP/7.5%TiO2 showed an interesting thermal stability and synergy effects on the peak of heat released rate (pHRR). Mechanisms were investigated following the analysis of residues and gas phase. Residues were analyzed by Raman spectroscopy, X-ray diffraction (XRD) and scanning electron microscope (SEM). Gases emitted during degradation were analyzed by Pyrolysisegas chromatographyemass spectrometry (PyeGCeMS). Comparison between cone calorimetry and pyrolysisecombustion flow calorimetry (PCFC) tests was a helpful tool for determining whether fire-retardancy mainly occurred by a physical or chemical pathway and for explaining the obtained results. Ó 2012 Elsevier Ltd. All rights reserved.

Keywords: Thermal properties Poly(methyl methacrylate) (PMMA) Flame retardant Nanocomposites Polyphosphates

1. Introduction It has been reported in a recent study that titanium dioxide (TiO2) nanoparticles modified the thermal degradation pathway of poly(methyl methacrylate) (PMMA) [1]. To explain the observed release of methanol, methacrylic acid and propanoic acid methyl ester, a mechanism taking place between the hydroxyl groups of the oxide surface and the ester function of PMMA was suggested. Moreover, the same authors observed that the commercial combination of polyphosphates (APP-based fire-retardant containing a small amount of melamine phosphate) could lead to interesting synergy effects and intumescence [2]. Through the present study, we intend to go further by substituting a part of TiO2 with two common fire-retardants (ammonium polyphosphate (APP) and melamine polyphosphate (MPP)) in a wide range of composition to provide cheap and efficient fire-retardant solutions for PMMA. The resulting system (APP/ MPP/TiO2) aims to be more efficient toward thermal and

* Corresponding author. Tel.: þ352 42 59 91 591. E-mail address: [email protected] (A. Laachachi). 0141-3910/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.polymdegradstab.2012.08.013

flammability properties through synergy effects. One purpose of this work is to depict the mechanisms occurring during the degradation in regards of the work performed on PMMAeTiO2. For that, a comparison of the peaks of heat released rate (pHRR) obtained in cone calorimetry and pyrolysisecombustion flow calorimetry (PCFC) could be helpful for determining if the involved fire retardation mechanisms mainly occurred by a physical or chemical way. Therefore we aim to present a scheme of mechanisms occurring in the condensed and in the gas phases as complete as possible.

2. Materials Poly(methyl methacrylate) (PMMA) (AcrigelÒ DH LE, Unigel Plàsticos e Mw ¼ 78,000 g mol1 determined by Gel Permeation Chromatography analysis) was used as the matrix. Nanometric titanium dioxide (AeroxideÒ TiO2 P25) with median particles size equal to 21 nm and specific surface area equal to 50 m2 g1 was provided by Evonik-Degussa GmbH. Ammonium polyphosphate (ExolitÒ AP 422) was given by Clariant (D50 ¼ 15 mm and phosphorus content of 31e32 wt%). Melamine polyphosphate (Melapur 200Ò, D98 ¼ 25 mm, 42e44 wt% nitrogen and 12e14 wt% phosphorus) was furnished by Ciba.

B. Friederich et al. / Polymer Degradation and Stability 97 (2012) 2154e2161

3. Nanocomposites preparation PMMA pellets were blended with ammonium polyphosphate (APP), melamine polyphosphate (MPP) and titanium dioxide (TiO2) nanoparticles in a Haake PolyLab 300 cm3 internal mixer at 225 C and 50 rpm. Prior to compounding, materials were dried at 80 C during at least 4 h in a stove. The mixing time was around 7 min, following 3 min of melt blending of PMMA (until the fall of viscosity). The total loading of the three additives was 15 wt%. The compositions of the mixtures of PMMA and (APP/MPP/metal oxide) ternary systems are presented in Table 1. The mixtures were then grinded depending on the tests performed on it. For cone calorimeter and thermal diffusivity measurements, samples were pressed under 55 bars at 240 C during 8 min using a hydraulic press from Carver. 4. Characterization 4.1. Nanocomposites morphology The dispersion of nanofillers in the PMMA matrix was checked by transmission electron microscopy (TEM, LEO 922 Omega) at 160 kV. The samples were 100 nm thick and were prepared with a LEICA EM FC6 cryo-ultramicrotome at 25 C. 4.2. Thermal properties 4.2.1. Thermogravimetric analysis (TGA) Thermogravimetric analyses (TGA) were performed with a STA 409 PC thermobalance from Netzsch operating under an air flow of 100 cm3 min1 in alumina crucibles (150 mL) containing around 15 mg. The runs were carried out in dynamic conditions at the constant heating rate of 10 C min1. 4.2.2. Laser flash analysis (LFA) The thermal diffusivity of PMMA and PMMA-APP/MPP/TiO2 nanocomposites was measured from 25 C to 170 C using a laser flash analysis (LFA) device from Netzsch (LFA 457 MicroflashÔ) (argon and air flows: 100 cm3 min1). PMMA-based nanocomposites were analyzed under inert atmosphere and residues under oxidative atmosphere (flows: 100 cm3 min1). For residues, measurements were made on disc-shape samples at 25 C. 4.3. Flammability properties 4.3.1. Cone calorimeter A cone calorimeter from Fire Testing Technology (FTT) was used to investigate flammability properties of PMMA-based nanocomposites. 100 100 4 mm3 sheets were exposed to a 35 kW m2 radiant heat flux and ignition by an electric spark. The fire was maintained with the radiant heat flux provided by the cone heater. Results were obtained from the average values of 2e3 samples for each formulation.

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4.3.2. Pyrolysisecombustion flow calorimetry (PCFC) In pyrolysisecombustion flow calorimetry (PCFC), a sample is first pyrolyzed in an inert gas stream (nitrogen) and then the volatile products are oxidized [3]. The experiment was performed on samples of 1e3 mg, using a Fire Testing Technology (FTT) calorimeter, at a heating rate of 1 C s1 from room temperature to 750 C in the pyrolysis zone. The combustion zone was set at 900 C under nitrogen/oxygen atmosphere (80/20 by volume) for a complete combustion of gases. 4.4. Analysis of residues 4.4.1. Residues morphology The morphology of residues was observed by using a FEI QUANTA FEG 200 environmental scanning electron microscope (ESEM). The working distance was around 10 mm and the acceleration voltage was equal to 15 kV. 4.4.2. Raman spectroscopy Raman spectroscopy studies were performed at room temperature with a Horiba Jobin-Yvon LabRam spectrometer. The excitation wavelength was 514.5 nm using an Arþ laser from Spectra Physics. A 50 objective lens was used for focusing the laser beam on the sample. Raman spectra were measured between 9000 and 100 cm1 with an acquisition time of 10 s. The final spectrum was the average of three spectra. All spectra were recorded with a 1800 lines mm1 network and a 1000 mm confocal hole. They were treated with the Labspec acquisition software. 4.4.3. X-ray diffraction (XRD) Wide angle X-ray diffraction (XRD) patterns were obtained by using a PANalytical X’Pert MPD X-ray diffractometer under a 45 kV voltage and with a 40 mA current. That device was equipped with A). The a copper anode emitting the radiation Ka (l ¼ 1.5418 diffraction tests were made with a q/2q diffractometer configuration. The crystalline phases were identified with the software X’Pert HighScore Plus 2.2d from PANalytical. 4.5. Analysis of the gas phase Pyrolysisegas chromatographyemass spectrometry (PyeGCe MS) analysis were performed with a CDS Pyroprobe 2000 coupled with an Agilent 6890 gas chromatograph equipped with a mass selective detector HP Agilent 5973 operating in the electron impact mode at 70 eV and a Optima-Wax column (high molecular weight compound of polyethylene glycol and diepoxide, 50 m length 0.25 mm diameter 0.25 mm film thickness). Helium was used at constant flow (1.1 mL min1) as the carrier gas. The sample weight was around 1 mg. The pyrolysis was performed during 60 s at 400 C (heating rate: 10,000 C s1 from 20 C to 400 C). 5. Results and discussion 5.1. Morphology of nanocomposites

Table 1 Compositions of PMMA-APP/MPP/TiO2 formulations with a total loading of 15 wt%. Formulation

APP

MPP

TiO2

PMMA-15%APP PMMA-15%MPP PMMA-15%TiO2 PMMA-7.5%APP/7.5%MPP PMMA-7.5%APP/7.5%TiO2 PMMA-7.5%MPP/7.5%TiO2 PMMA-5%APP/5%MPP/5%TiO2

1 0 0 1/2 1/2 0 1/3

0 1 0 1/2 0 1/2 1/3

0 0 1 0 1/2 1/2 1/3

TEM analysis of PMMA-APP/MP/TiO2 nanocomposites was performed in order to ascertain the dispersion and the distribution of titanium dioxide nanoparticles in PMMA, in the presence of polyphosphates. Fig. 1 shows a micrograph obtained for PMMA-5%APP/ 5%MPP/5%TiO2 compared to the one obtained for PMMA-5%TiO2. If TiO2 alone disperses well into PMMA, it seems difficult to differentiate the phosphorated additives from titanium oxide particles. However, their presence does not affect the dispersion of TiO2 nanoparticles in a negative way.

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Fig. 1. TEM micrograph of PMMA-5%TiO2 (a) [1] and PMMA-5%APP/5%MPP/5%TiO2 (b).

5.2. Fire behavior The fire behavior of PMMA-APP/MPP/TiO2 ternary system was studied by cone calorimetry under an incident flux of 35 kW m2. The values of the time to ignition (TTI), time of flame out (TOF), peak of heat released rate (pHRR), total heat released (THR), smoke emission (TCOR) alongside with the mass loss of the sample are presented in Table 2. The combustion time corresponds to the difference between TOF and TTI, the pHRR decrease is the decrease from PMMA and the fire performance index (FPI) is the ratio of TTI and pHRR. THR decreases in comparison with PMMA when adding additives into the polymer. TTI tends to increase except for PMMA-7.5% APP/7.5%MPP and PMMA-7.5%MPP/7.5%TiO2. The highest TTI are for PMMA-15%TiO2 and PMMA-7.5%APP/7.5%TiO2 (respectively 88 and 75 s compared to 62 s for PMMA). PMMA-7.5%APP/7.5%TiO2 and PMMA-5%APP/5%MPP/5%TiO2 owns the longest combustion time of the whole system (906 and 935 s). pHRR decreases when additives are loaded into PMMA; the lowest pHRR are observed for PMMA-15%MPP (reduction of 51%), PMMA-7.5%APP/7.5%MPP (52%) and PMMA-7.5%APP/7.5%TiO2 (52%). PMMA-7.5%APP/7.5%TiO2 is a very interesting mixture, because it has the highest FPI, the lowest THR, one of the longest combustion time and a synergy effect concerning the pHRR (pHRR decrease of 52% with 257 kW m2). The particularity of that formulation is also observable when displaying the mass loss rate of the sample versus time. Indeed titanium dioxide brings about the strongest slow down of the mass loss rate as depicted in Fig. 2. It is originating

from the modification of the kinetics of polymer degradation, which can be related to the increase of the time of combustion for PMMA. Indeed, that mixture presents the highest time of combustion (831 s). Fire-retardants are capable to have an action toward fireresistance through chemical mechanisms (radical reactions, modification of the degradation path, dilution of inert gases, .), physical mechanisms (barrier effect, intumescence, .) or through both of them. It is possible to determine by which pathway (physical or chemical) fire protection mainly takes place by comparing the decrease of pHRR obtained by PCFC and by cone calorimeter tests [5]. This can be done as, due to its features, PCFC takes only into account mechanisms occurring by chemical ways leading to flammable gases, gases released after pyrolysis being completely burnt whereas the cone calorimeter test detects both mechanisms of fire-retardants taking place by chemical or physical modes of action. Table 3 presents the pHRR values measured, their decrease compared to PMMA and the ratio of chemical and physical mechanisms deduced from these data. As shown in Table 3, when APP, MPP and TiO2 are loaded alone into PMMA with a loading of 15 wt%, the fire-resistance mechanism mainly taking place is physical (respectively 77, 61 and 71%), since the pHRR decrease is much more important by cone calorimeter than by PCFC. When combining APP and MPP, the protection of that formulation from fire still mainly takes place by a barrier effect. The physical effect of PMMA-15%TiO2 is probably due to a barrier effect [6]. The substitution of that metal oxide by APP and/or MPP leads to a decrease of the part of the physical mechanisms.

Table 2 Cone calorimetry data of PMMA-APP/MPP/TiO2 system (5% standard deviation).

TTI(s) TOF(s) Combustion time (s) pHRR (kW m2) pHRR decrease (%) Fire Performance Index (kW1 m2 s) THR (MJ m2) Final residues (%) TCOR (g kg1)

PMMA

PMMA-15%TiO2

PMMA-15% APP

PMMA-15%MPP

PMMA-7.5%APP/7.5%MPP

PMMA-7.5%APP/7.5%TiO2

PMMA-7.5% MPP/7.5% TiO2

PMMA-5% APP/5% MPP/5% TiO2

62 339 337 533 / 0.116

88 608 520 347 35 0.254

63 560 497 345 35 0.183

67 675 608 260 51 0.258

58 710 652 255 52 0.227

75 906 831 257 52 0.292

59 775 716 278 48 0.212

65 935 770 271 49 0.240

117 0 7

100 15 10

100 12 10

99 6 21

103 10 7

93 16 18

99 11 13

99 11 10


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Fig. 2. Mass loss for formulations based on titanium dioxide measured by cone calorimeter (heat flux: 35 kW m2).

5.3. Thermal properties

The thermal diffusivity of PMMA alone could not be obtained after combustion, because the polymer depolymerizes at more than 95% and left no usable residues. As shown by Table 4, the addition of additives (TiO2, APP and/or MPP) into PMMA resulted in the increase of thermal diffusivity of initial materials compared to the neat polymer. In these conditions, upon degradation, heat transfer is more efficient and temperature gradients inside the sample are decreasing. Therefore, the better heat dissipation ensuing in the sample lowers the surface temperature retarding thermal decomposition which tends to increase the time to ignition of the material. After combustion, the thermal diffusivity is always higher than for initial samples. It should be noticed that the values after combustion were obtained on pellets of pressed residues which mean that they are intrinsic values of the material not taking into account the porosity and alveolar character of the intumescent residues. Apart from any consideration of the intumescent protective character developing during combustion, for a given composition, the observed increase of thermal diffusivity before and after combustion can be interpreted by the formation of a more and more heat dissipating residual layer which can contributes to slow down the degradation of the polymer. So, the improvement of the fire behavior can be tentatively ascribed to a better dissipation of heat in the material allowing to slow down the degradation combined to a better protection from heat due to the foaming structure of the residues.

5.3.1. Thermal analysis Thermogravimetric analysis was performed under air on PMMA and on PMMA-APP/MPP/TiO2 from room temperature to 900 C with a heating rate of 10 C min1. This was done in order to highlight the effect of titanium dioxide, APP and MPP on the thermal stability of the polymer. In Fig. 3, it is remarkable that each additive has not the same effect on the thermal stability of PMMA according to T50% values: APP and MPP do not improve that parameter, whereas titanium dioxide significantly enhance it with an improvement of 29 C for 15%TiO2 compared to PMMA and 25e27 C compared to 15%APP or 15%MPP. Moreover the more metal oxide is substituted to phosphorus compounds, the more the thermal stability is improved. But the highest value obtained for the thermal stability is for PMMA-7.5%APP/7.5%TiO2. This is also obvious in Fig. 3 which presents the TGA curves for the ternary system. Fig. 3 shows that the thermal decomposition undergoes an inhibiting effect (observable at T10%) increasing when the amount of metal oxide increases. This shows that the nature of the interphase region affects the degradation pathway of the polymer. 5.3.2. Thermal diffusivity measurement For PMMA containing nanometric metal oxides and according to our previous work [4], the thermal degradation rate is linked to the thermal diffusivity of the sample. This led us to measure the thermal diffusivity of the ternary system samples. Table 4 presents the obtained results for PMMA and mixtures of PMMA-APP/MPP/ TiO2 measured at room temperature before and after combustion in order to verify the effect of degradation on the heat dissipation capacity of samples.

5.4. Investigation of fire-resistance mechanisms 5.4.1. Analysis of residues In order to present a complete scheme of fire-resistance mechanisms of the ternary system (APP/MPP/TiO2) in PMMA, the condensed phase (residues) obtained after degradation and the

Table 3 pHRR decreases by PCFC and cone calorimeter tests on (APP/MPP/TiO2) ternary system in PMMA and ratio of chemical and physical mechanisms. PCFC

PMMA PMMA-15%APP PMMA-15%MPP PMMA-15%TiO2 PMMA-7.5%APP/7.5%MPP PMMA-7.5%APP/7.5%TiO2 PMMA-7.5%MPP/7.5%TiO2 PMMA-5%APP/5%MPP/5%TiO2

Cone calorimeter

pHRR (kW m2)

pHRR decrease (%)

pHRR (kW m2)

pHRR decrease (%)

Chemical mechanisms (%)

Physical mechanisms (%)

402 371 323 360 361 273 326 297

/

533 345 260 347 255 257 278 271

/ 35 51 35 52 52 48 49

/ 23 39 29 19 62 40 53

/ 77 61 71 81 38 60 47

8 20 10 10 32 19 26

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gases emitted during the degradation have been analyzed. Fig. 4 presents the residues obtained after cone calorimeter experiments. It is remarkable in Fig. 4 that PMMA-15%MPP is the sample exhibiting the most important intumescent residue due to the main release of gaseous melamine and of some ammonia originating from the product of deamination reaction of melamine. This helps to protect the material against flames, heat and oxygen [7]. But, the visual observation of residues presented in Fig. 4 is not sufficient to conclude on the mechanisms occurring during degradation. Residues were therefore analyzed by scanning electron microscopy (SEM), Raman spectroscopy and X-ray diffraction (XRD) for getting a better understanding of phenomena occurring in the solid phase upon degradation.

Fig. 3. TGA curves of PMMA and its composites with TiO2, APP and MPP at 15 wt% (a) and compared to APP/MPP/TiO2 ternary system in PMMA (b) under air (heating rate: 10 C min1).

Table 4 Thermal diffusivity of PMMA and PMMA-APP/MPP/TiO2 system, before and after combustion in the cone calorimeter. Before combustion Thermal diffusivity (mm2 s1)

PMMA PMMA-7.5%APP/7.5%TiO2 PMMA-7.5%MPP/7.5%TiO2 PMMA-5%APP/5%MPP/5%TiO2

0.107 0.127 0.130 0.144

0.002 0 0.001 0.001

After combustion / 0.184 0.004 0.255 0.001 0.204 0.015

5.4.1.1. Residues morphology. Residues were analyzed by scanning electron microscope (SEM) (Fig. 5). They are not continuous and present holes which appear during the cone calorimeter test. These holes give the possibility to gases to be released upon the degradation of the samples. Thus, they lower the physical barrier effect and the fire-resistance of the condensed phase. The amount of holes is between the amount obtained for PMMA-APP/MPP/AlOOH and PMMA-APP/MPP/Al2O3 ternary systems [8] and like for these systems, it is possible to link the amount of holes and the pHRR decrease obtained through cone calorimeter and PCFC tests (fire-resistance). Indeed Ref. [8] shows that formulations containing boehmite (AlOOH) and APP and/or MPP lead to residues with a low number of holes and modes of action mainly taking place by a physical way (respectively equal to 64%, 49% and 62% for PMMA-7.5%APP/7.5%AlOOH, PMMA-7.5%MPP/ 7.5%AlOOH and PMMA-5%APP/5%MPP/5%AlOOH). On the contrary, formulations containing alumina (Al2O3) and APP and/or MPP lead to residues with a high amount of holes and modes of action mainly taking place by a chemical way (respectively equal to 64%, 56% and 63% for PMMA-7.5%APP/7.5%Al2O3, PMMA-7.5%MPP/7.5%Al2O3 and PMMA-5%APP/5%MPP/5%Al2O3). Formulations containing titanium

Fig. 4. Cone calorimeter residues of PMMA-APP/MPP/TiO2.


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Fig. 5. Micrographs of residues for PMMA-7.5%APP/7.5%TiO2 (a), PMMA-7.5%MPP/7.5%TiO2 (b) and PMMA-5%APP/5%MPP/5%TiO2 (c).

dioxide and APP and/or MPP exhibit a number of holes intermediate between the two systems studied in Ref. [8] and at the same time, the fire-retardants act by both physical and chemical ways (Table 3). 5.4.1.2. Raman spectroscopy. In Raman spectroscopy, aromatic cycles are characterized by two peaks: D- and G-bands at respectively 1350 and 1580 cm1. D-band represents the disordered graphite like clusters of hexagonal cycles. It is associated to the vibrational mode A1g and it is commonly named the “default band”. The G-band can be assigned to the ordered graphite stemming from ordered hexagonal cycles which consist in carbon atoms connected with sp2 bonds. That band corresponds to the vibrational mode E2g [9e11]. Products containing aromatic cycles are known for having high thermal diffusivity values. Xie et al. [12] showed in fact that thermal diffusivity of carbon nanotubes can reach 4.6 cm2 s1. Therefore, the formation of aromatic hydrocarbons (carbonaceous layer) during the degradation of samples helps to increase thermal diffusivity and to dissipate the heat in residues. But the disappearance of the organic phase and the resulting concentration of metal oxides also leads to the increase of thermal diffusivity. Beside the formation of aromatic hydrocarbons, Fig. 6 shows that a part of titanium dioxide reacted with phosphoric acid (H3PO4), originating from the degradation of phosphorus compounds (APP and MPP), to give titanium pyrophosphate (TiP2O7) according to the following reaction (1).

TiO2 þ 2H3 PO4 /TiP2 O7 þ 3H2 O

(1)

TiP2O7 was only formed when APP and TiO2 were combined, since it was not detected when MPP and TiO2 were combined into PMMA (without APP). The assignment of the main Raman bands allowing to identify titanium pyrophosphate was achieved from literature data [13] as well as the two crystalline forms of TiO2 [14,15]. The formation of titanium pyrophosphate was already observed in a previous work performed with a commercial fire-retardant based on APP [2]. Its role is not clearly established but it could contribute to the densification of the residue and/or to the cooling of the system if its reaction of formation is endothermic provided its amount is sufficient to ensure a sufficient effect. Unfortunately, no data are available in the literature allowing to estimate the enthalpy of formation of TiP2O7. 5.4.1.3. X-ray diffraction (XRD). Fig. 7 presents the X-ray diffraction (XRD) pattern of the residues obtained after the cone calorimeter test on PMMA-7.5%APP/7.5%TiO2. X-ray diffraction technique confirmed Raman spectroscopy tests, because both TiP2O7 and remaining TiO2 could be detected (Fig. 7). Both compounds were also detected in PMMA-5%APP/5% MPP/5%TiO2 residues, but not in PMMA-7.5%MPP/7.5%TiO2 residues. Therefore MPP does not lead to the formation of TiP2O7 when combined, as shown by Raman spectroscopy, or the amount was too low for being detected.

Fig. 6. Raman spectra of PMMA-APP/MPP/TiO2 ternary system’s residues.

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Fig. 7. XRD spectrum on residues of PMMA-7.5%APP/7.5%TiO2.

5.4.2. Analysis of the gas phase The gases emitted during burning were analyzed by Pyrolysise gas chromatographyemass spectrometry (PyeGCeMS), for completing the degradation scheme presented for the condensed phase. In order to compare the variation of the pyrolysates composition as a function of the amount of APP, MPP and metal oxides nanoparticles, the total ion current (TIC) chromatograms was

normalized to mass unity for each formulation. Since methyl methacrylate monomer is always produced in high quantities saturating the mass detector, the ion current corresponding to the following molecular ions: m/z ¼ 32 (methanol) and m/z ¼ 86 (methacrylic acid or MAA) was extracted from TIC to avoid any interference. After integration, the evolution of the peak areas of these ions was compared for each formulation of the ternary system. In a previous paper [1], Laachachi et al. proposed a mechanism occurring between the surface of titanium dioxide (eOH groups) and ester functions of PMMA. They observed that this reaction leads to the release of methanol, methacrylic acid and propanoic acid methyl ester. In the present paper, since APP and MPP were added into the system, we have ascertained whether that mechanism is maintained or another reaction is involved. That is the reason why the amount of these three products was measured following the pyrolysis of the samples. On the x-axis of Fig. 8, the loading of metal oxide increases from 0 to 15 wt% and the loadings of APP and MPP decreases at the same time. Thus, formulations of the ternary system PMMA-APP/MPP/ metal oxide are placed at the loading of the metal oxide. Since it is difficult to differentiate PMMA-7.5%APP/7.5% oxide and PMMA7.5 MPP/7.5% oxide, which have the same oxides loading, their locations are specified. Fig. 8 shows that the release of methanol, methacrylic acid and propanoic acid methyl ester increases when the titanium dioxide loading increases. The trend is the same as for PMMAeTiO2 system (Fig. 8a and b). As in Ref. [1], the amount of propanoic acid methyl ester is very low (Fig. 8c). Therefore, it can be assumed that a reaction between TiO2 and PMMA is present even when TiO2 is combined with APP and/or MPP.

Fig. 8. Ion current integrated areas of m/z ¼ 32 (methanol) (a), m/z ¼ 86 (methacrylic acid) (b) and m/z ¼ 88 (propanoic acid methyl ester) (c) for PMMA-APP/MPP/TiO2.


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Fig. 9. Reaction of cyclization between PMMA and the polyphosphoric acid in the ternary system PMMA-APP/MPP/TiO2.

Moreover, according to Camino et al. [16], there is also a reaction between PMMA and one of the degradation products of polyphosphates (polyphosphoric acid). That reaction, presented in Fig. 9, leads to the release of methanol. The anhydride obtained in Fig. 9 could degrade into methacrylic acid. The combination of TiO2 with polyphosphates mainly led to these two reactions and that could explain why the proportion of chemical mechanisms become more important than when the metal oxide is alone in PMMA (Table 3). 6. Conclusion The substitution of a part of metal oxide with flame-retardants containing nitrogen and phosphorus leads to the improvement of the fire-resistance and thermal properties of PMMA. We have observed that PMMA-7.5%APP/7.5%TiO2 is the best formulation within the ternary system, since it leads to the highest TTI (75 s), the best pHRR decrease (52%) and the lowest THR compared to PMMA. That formulation also exhibits the best thermal stability increase (33 C compared to PMMA). Our main goal was to investigate the reasons of these improvements within the ternary system PMMA-APP/MPP/TiO2. For that, both residues and gases emitted were analyzed. Two main mechanisms could take place: a reaction between titanium dioxide and PMMA and another one (cyclization) between APP and/or MPP and PMMA. In PMMA-15%TiO2, the ratio of physical mechanisms (71%) is more important than chemical mechanisms. But PCFC tests showed that when TiO2 was combined with polyphosphates, the ratio of chemical mechanisms became preponderant. In the residues, all compounds could be identified by XRD and Raman spectroscopy: - Titanium pyrophosphates (TiP2O7) which comes from the reaction between TiO2 and phosphoric acid (a degradation product of polyphosphates); - Remaining titanium dioxide after combustion; - Aromatic cycles which participate to a better heat dissipation, preventing the heat to build-up at the surface of the sample. That property contributes to slow down the thermal degradation of the sample. A link between the presence of holes in the residues and the fire-retardant capacity could be deduced from the SEM observations.

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