Real Time Analysis of Volatile Organic Compounds from

Jul 2, 2009 - Laboratoire d'Ingénierie des Matériaux, Arts et Métiers ParisTech/CNRS (UMR 8006), 151 Bd de l'Hôpital,. F-75013 Paris, AlyXan, Universi...
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Anal. Chem. 2009, 81, 6013–6020

Real Time Analysis of Volatile Organic Compounds from Polypropylene Thermal Oxidation Using Chemical Ionization Fourier Transform Ion Cyclotron Resonance Mass Spectrometry Salah Sarrabi,† Xavier Colin,† Abbas Tcharkhtchi,† Michel Heninger,‡ Julien Leprovost,‡ and He´le`ne Mestdagh*,§ Laboratoire d’Inge´nierie des Mate´riaux, Arts et Me´tiers ParisTech/CNRS (UMR 8006), 151 Bd de l’Hoˆpital, F-75013 Paris, AlyXan, Universite´ Paris-Sud, Baˆt. 207B, F-91405 Orsay, and Laboratoire de Chimie Physique, Universite´ Paris-Sud/CNRS (UMR 8000), Baˆt. 350, F-91405 Orsay, France The volatile organic compounds emitted during polypropylene thermal oxidation at constant temperature were analyzed in real time using proton transfer reaction associated with Fourier transform ion cyclotron resonance mass spectrometry. Four major compounds were identified: acetone, formaldehyde, acetaldehyde, and methylacrolein. The time dependence of the emissions showed a complex profile consisting successively in an induction period, a stationary state with low emission rate, and final rapid degradation of the sample. The formaldehyde mixing ratio increases by a factor of 3 between the stationary state stage and the fast degradation stage, while the other products remain in constant proportions. The kinetic and mechanistic implications of these results are discussed. Oxidative degradation of polymers results from aging at low temperature for a long term (in use conditions) or at high temperature in the molten state (in processing conditions). The corresponding oxidation processes are important to understand and control since they affect the mechanical properties of the material.1 Another concern about polymer thermal oxidation lies in emission of potentially toxic volatile organic compounds (VOCs). Polypropylene (PP) thermal aging has provoked a large amount of elaborate research works in the past half-century. Most of these works consisted in the study of the PP degradation state, after thermal exposure in a well-controlled oxidizing atmosphere, by various analytical methods. The main objective was to better understand the complex mechanism of PP thermal oxidation and, eventually, to propose a global reaction pathway. The mostly used techniques were hydroperoxide titration,2-4 infrared emission5 or absorption6-13 spectroscopy (FT-IR), nuclear magnetic resonance * To whom correspondence should be addressed. E-mail: helene.mestdagh@ lcp.u-psud.fr. † Arts et Me´tiers ParisTech/CNRS (UMR 8006). ‡ AlyXan, Universite´ Paris-Sud. § Laboratoire de Chimie Physique, Universite´ Paris-Sud/CNRS (UMR 8000). (1) Colin, X.; Verdu, J. C. R. Chim. 2006, 9, 1380–1395. (2) Mair, R. D.; Graupner, A. J. Anal. Chem. 1964, 36, 194–204. (3) Carlsson, D. J.; Lacoste, J. Polym. Degrad. Stab. 1991, 32, 377–386. (4) Gijsman, P.; Hennekens, J.; Vincent, J. Polym. Degrad. Stab. 1993, 42, 95– 105. 10.1021/ac802353r CCC: $40.75  2009 American Chemical Society Published on Web 07/02/2009

(NMR),14,15 and thermogravimetric analysis (TGA).16-18 Manometric measurement of oxygen absorption19 and measurement of chemiluminescence emission during oxidation20,21 are particularly interesting methods due to their high sensitivity, allowing the most initial stages of oxidation to be studied. While the quoted techniques are focused on the remaining polymeric condensed phase, analysis of the volatile compounds emitted by PP oxidative degradation is able to give valuable information, which complements the condensed-phase information in supporting the development of kinetic and mechanistic models. The volatile products emitted in PP thermal oxidation have been analyzed using gas chromatography (GC)17,22 often coupled with mass spectrometry (GC/MS).23-25 They include CO, CO2, and (5) George, G. A.; Celina, M.; Vassallo, A. M.; Cole-Clarke, P. A. Polym. Degrad. Stab. 1995, 48, 199–210. (6) Luongo, J. P. J. Appl. Polym. Sci. 1960, 3, 302–309. (7) Chien, J. C. W.; Vandenberg, E. J.; Jabloner, H. J. Polym. Sci., Part A-1 1968, 6, 381–392. (8) Chien, J. C. W. Polymer Stabilization; J. Wiley & Sons: New York, 1972; Chapter 5, pp 95-112. (9) Adams, J. H. J. Polym. Sci., Part A-1 1970, 8, 1077–1090. (10) Severini, F.; Gallo, R.; Ipsale, S. Polym. Degrad. Stab. 1988, 22, 185–194. (11) Lacoste, J.; Vaillant, D.; Carlsson, D. J. J. Polym. Sci., Part A: Polym. Chem. 1993, 31, 715–722. (12) Teisse`dre, G.; Pilichowski, J. F.; Lacoste, J. Polym. Degrad. Stab. 1994, 45, 145–153. (13) Gugumus, F. Polym. Degrad. Stab. 1998, 62, 235–243. (14) Vaillant, D.; Lacoste, J.; Dauphin, G. Polym. Degrad. Stab. 1994, 45, 355– 360. (15) Thornberg, S. M.; Bernstein, R.; Irwin, A. N.; Derzon, D. K.; Klamo, S. B.; Clough, R. L. Polym. Degrad. Stab. 2007, 92, 94–102. (16) Girois, S.; Audouin, L.; Delprat, P.; Verdu, J. Polym. Degrad. Stab. 1996, 51, 133–141. (17) Hayashi, J.-I.; Nakahara, T.; Kusakabe, K.; Morooka, S. Fuel Process. Technol. 1998, 55, 265–275. (18) Rychly, J.; Matisova-Rychla, L.; Csmorova, K.; Achimsky, L.; Audouin, L.; Tcharkhtchi, A.; Verdu, J. Polym. Degrad. Stab. 1997, 58, 269–274. (19) Wise, J.; Gillen, K. T.; Clough, R. L. Polym. Degrad. Stab. 1995, 49, 403– 418. (20) George, G. A. In Luminescence Techniques in Solid State Polymer Research; Zlatkevitch, L., Ed.; Marcel Dekker: New York, 1989; p 93. (21) Eriksson, P.; Reitberger, T.; Stenberg, B. Polym. Degrad. Stab. 2002, 78, 183–189. (22) Kiryushkin, S. G.; Mar’in, A. P.; Shlyapnikov, Y. A. Polym. Sci. U.S.S.R. 1980, 22, 1570–1574. (23) Barabas, K.; Iring, M.; Laszlo-Hedvig, S.; Kelen, T.; Tudos, F. Eur. Polym. J. 1978, 14, 405–407. (24) Hoff, A.; Jacobsson, S. J. Appl. Polym. Sci. 1984, 29, 465–480.

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acetone according to all the reports. In addition, a variety of VOCs have been reported: acetaldehyde and methylacrolein, along with C4 and C5 carbonyl compounds;23 acetaldehyde, acetone, and C1-C3 hydrocarbons;22 formaldehyde, acetaldehyde, acetic acid, methacrolein, and 2-pentanone, with other carbonyl compounds, acids, alcohols, and hydrocarbon traces;24 methanol and acetaldehyde;17 C4-C6 saturated and unsaturated ketones and C1-C4 carboxylic acids.25 Use of 18O-labeled O226 and of 13C selectively labeled polypropylenes15,25 helped to specify the origin of some of the products. Photooxidation of PP27 or its corresponding hydroperoxides28 follows a slightly different course, leading to methanol, acetone, and acetic acid as volatile products. Kinetic modeling of PP thermal oxidation has been the subject of numerous works aimed at the improvement of the mechanistic model, by comparing predicted and experimental results concerning the time dependence of a variety of measurable physical and chemical properties during PP thermal oxidation.18,29,30 This oxidative degradation process is known to follow complex kinetics, and to be extremely sensitive to reaction conditions such as temperature and O2 partial pressure.31,32 In some specific cases (for instance, in PP-containing catalytic residues), the kinetics may be more complicated, consisting in the propagation of “infectious” sites across the sample.33 Among the relevant properties, the chemical composition of the VOC emissions is likely time-dependent, and its detailed knowledge may be very helpful to predict the prevailing mechanisms. With the chromatographic techniques used in most cases for VOC emission analysis, it is difficult to obtain real time and quantitative following of the emissions. At best, the global amount of each compound could be recorded for different degradation times.23 A particularly suitable analytical tool for complex mixtures of VOCs diluted in air is proton transfer reaction associated with Fourier transform ion cyclotron resonance mass spectrometry (PTR-FTICR). This technique is based on analyte chemical ionization by proton transfer reaction from H3O+, and detection of the resulting ions with an FTICR mass analyzer. Each analyte A of molecular mass M is thus detected as the corresponding protonated ion AH+, m/z ) M + 1. This soft and selective ionization technique allows detection of most VOCs, without ionization of air major components, which have lower proton affinities (PAs) than water. It has been widely used in PTRMS34 (25) Bernstein, R.; Thornberg, S. M.; Assink, R. A.; Irwin, A. N.; Hochrein, J. M.; Brown, J. R.; Derzon, D. K.; Klamo, S. B.; Clough, R. L. Polym. Degrad. Stab. 2007, 92, 2076–2094. (26) Philippart, J. L.; Gardette, J. L. Polym. Degrad. Stab. 2001, 73, 185–187. (27) Philippart, J.-L.; Posada, F.; Gardette, J.-L. Polym. Degrad. Stab. 1995, 49, 285–290. (28) Commereuc, S.; Vaillant, D.; Philippart, J. L.; Lacoste, J.; Lemaire, J.; Carlsson, D. J. Polym. Degrad. Stab. 1997, 57, 175–182. (29) Rincon-Rubio, L. M.; Fayolle, B.; Audouin, L.; Verdu, J. Polym. Degrad. Stab. 2001, 74, 177–188. (30) Sarrabi, S.; Colin, X.; Tcharkhtchi, A. J. Appl. Polym. Sci., in press. (31) Richaud, E.; Farcas, F.; Bartolomeo, P.; Fayolle, B.; Audouin, L.; Verdu, J. Polym. Degrad. Stab. 2006, 91, 398–405. (32) Richaud, E.; Farcas, F.; Fayolle, B.; Audouin, L.; Verdu, J. Polym. Degrad. Stab. 2007, 92, 118–124. (33) Celina, M.; Clough, R. L.; Jones, G. D. Polym. Degrad. Stab. 2006, 91, 1036–1044. (34) Lindinger, W.; Hansel, A.; Jordan, A. Int. J. Mass Spectrom. Ion Processes 1998, 173, 191–241.

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Table 1. Some Characteristics of the As-Received PP Powdera characteristic

technique (conditions)

value

unit

Mw Tf Tc isoT Lc

SEC DSC (N2, 10 °C · min-1) DSC (N2, 10 °C · min-1) IR (transmission) DSC (N2, 10 °C · min-1)

290 167 110 66 7.3

kg · mol-1 °C °C % nm

a Mw ) weight average molar mass, Tf ) melting point, Tc ) crystallization temperature, isoT ) isotacticity, and Lc ) crystalline lamella thickness.

and SIFT35 instruments. The specific advantage brought by FTICR mass detection is its remarkable mass resolution and accuracy, allowing reliable determination of the analyte molecular formulas. In addition, FTICR mass spectrometry is based on broad-band detection, meaning simultaneous analyte monitoring over the whole mass range. Finally, PTR-FTICR allows quantitative analysis and real time monitoring of the sample composition.36 Compact FTICR mass spectrometers suited for analytical applications have been recently developed by AlyXan and the LCP group.37 These instruments, such as the prototype BTrap used in this work, are based on structured permanent magnets and optimized for chemical ionization. The aim of the present paper is real time characterization and quantification of the VOCs emitted during thermal degradation of an unstabilized PP film at high, constant temperature, using PTR-FTICR. The results are discussed in comparison with those of other analytical methods. EXPERIMENTAL SECTION Sample Preparation. Experiments were conducted on an unstabilized and unfilled, mainly isotactic (66%) PP powder provided by PRIEX RESINS. Before the tests, the as-received PP powder was characterized by IR spectrophotometry, differential scanning calorimetry (DSC), and steric exclusion chromatography (SEC). The main results obtained are summarized in Table 1. Thick PP plates of 4 mm thickness were processed by a DK CODIM 175-400 injection press. Injection conditions were the following: The mold temperature was set to 20 °C, the melt polymer temperature to 220 °C, the hold pressure at the nozzle to 30 MPa, the holding time to 10 s, and the total cooling time to 40 s. After injection, the plates were characterized by SEC to confirm that the polymer did not involve a preliminary degradation during processing. Then the PP plates were microtome cut into thin slices of 40 µm thickness, perpendicularly to the sample surfaces. Small film pieces (1.5, 2.8, 4.4, and 7.0 mg) were weighed before introduction into the oven. Experimental Setup. The experimental setup is schematized in Figure 1. The sample was introduced into an oven and swept by a controlled air flow, which was continuously sampled by the mass spectrometer. The oven temperature was stabilized to 256 °C. The sample was put in a small quartz vessel and introduced into the preheated (35) Smith, D.; Spanel, P. Mass Spectrom. Rev. 2005, 24, 661–700. (36) Dehon, C.; Gauzere, E.; Vaussier, J.; Heninger, M.; Tchapla, A.; Bleton, J.; Mestdagh, H. Int. J. Mass Spectrom. 2008, 272, 29–37. (37) Heninger, M.; Clochard, L.; Mestdagh, H.; Boissel, P.; Lemaire, J. Spectra Anal. 2006, 248, 44–49.

Figure 1. Experimental setup for real time analysis of VOCs emitted from PP thermal oxidation.

oven at zero time. A continuous gas flow generated by the GasMix device (AlyTech) was driven through the oven. For all the experiments the flow was 74 mL/min. The transfer line was continuously heated at 80 °C. A part of the outlet flow was taken through the leak valve, which was adjusted to reduce the gas pressure to a few torrs. A second stage of pressure reduction was made by a dimensioned capillary tube (0.02 in. i.d.) being heated, allowing the appropriate pressure of ca. 4 × 10-5 Torr in the cell to be reached. The capillary tube was continuously heated to avoid condensation. For estimation of the real degradation time corresponding to a mass spectrum recorded at a given time from introduction of the sample into the oven, the transport time of the gas from the sample to the mass spectrometer inlet has to be taken into account. The volume between the sample position and the capillary tube is ca. 77 mL. With a gas flow of 74 mL/min, the resulting transport time is 1.04 min. In the following this transport time is systematically subtracted from the time measured from sample introduction. B-Trap Mass Spectrometer. B-Trap Specifications. The design of the B-Trap spectrometers is close to that of the compact mass spectrometer MICRA, described previously.38 They are optimized for mixture analysis and trace analysis using chemical ionization, and specially designed to be compact and transportable. Their detailed design and performances will be described in a forthcoming paper. The spectrometer used in this work weighs less than 150 kg and fits into a volume of 0.72 m3 (L × l × h: 1.0 m × 0.60 m × 1.2 m). Its different parts are briefly described below. Magnet: FTICR performance is strongly dependent on the magnetic field strength, homogeneity, and stability. B-Trap’s permanent magnet, providing a 1 T field, is made of FeNdB, measuring 12 cm in length, with an internal bore of 5 cm and weighing 17 kg. ICR cell: The ICR cell implemented in the B-Trap is cubic, with internal dimensions 2 × 2 × 2 cm. A tungsten filament, fitted on one of the trapping plates, allows electron impact generation of the primary ions needed for chemical ionization methods such as PTR. Vacuum system and pressure measurement: The ICR cell is pumped by a 70 L/s turbomolecular pump, backed by a membrane (38) Mauclaire, G.; Lemaire, J.; Boissel, P.; Bellec, G.; Heninger, M. Eur. J. Mass Spectrom. 2004, 10, 155–162.

pump, giving a nominal base pressure of a few 10-9 Torr. A second turbomolecular pump (30 L/s) is used to evacuate the gas inlet system. The pressure in the cell zone was measured using an ionization gauge calibrated for air. Such gauges are sensitive to a magnetic field. Comparison of the gauge indications inside and outside the magnet led to correction of the pressure read by a 1.88 multiplying factor when the cell was inside the magnet. In the present work the corrected cell pressure was typically 1.8 × 10-5 Torr. Gas inlet system: Two gas inlet lines are used to introduce gas into the mass spectrometer. The first one is used for introduction of water necessary to generate the H3O+ precursor ions; the second one allows introduction of the sample as described above. Each gas inlet line includes a three-way pulsed valve that directs the gas flow either to the mass spectrometer main vacuum chamber or to a gas inlet evacuation line. This device avoids pressure surges at the beginning of each gas pulse into the spectrometer. After degassing of the liquid water, the water vapor is sent through a leak valve to regulate the pressure, and to the three-way valve. Data acquisition system: The mass spectrometer is controlled by a custom-built detection unit based on a standard PC fitted with commercial PCI cards for excitation/detection of the ions. The system is close to that described for the MICRA analyzer. Analytical Sequence. The H3O+ ions are produced in the cell by electron ionization of H2O at a typical pressure of 10-5 Torr using a 70 eV electron beam, followed by ion-molecule reaction of the resulting H2O+ ions: H2O+ + H2O f H3O+ + OH The succession of precursor ion preparation, chemical ionization reaction, and ion detection leading to the mass spectrum is carried out according to the typical programmed operating sequence below. (1) Introduction of H2O steam (a few 10-6 Torr) (20 ms). (2) Electron ionization of H2O (2 ms). (3) Reaction time for H3O+ formation (200 ms). (4) Introduction of the neutral sample (a few 10-5 Torr). During this step, the analytes are ionized by proton exchange. The duration of this step is the reaction time. It is adjusted between 50 ms and 1 s, depending on the introduction pressure and the concentrations to be detected. Its optimum value corresponds to consumption of ca. 30% of the H3O+ reactant, which represents a compromise between two needs, a high signal-to-noise ratio favored by a high reaction yield and negligible secondary reactions obtained for a low reaction yield.36 (5) Delay for complete pumping of the cell (>200 ms). The vacuum quality into the ICR cell is essential for an optimal detection. As a consequence, the neutral gas injected must be pumped as efficiently as possible. The delay used allows the pressure to fall below 10-7 Torr, which ensures a satisfying detection. (6) RF excitation of the ions (0.5 ms). The ions are put into circular motion by using RF pulses. Each ion rotates according to its cyclotron frequency. Analytical Chemistry, Vol. 81, No. 15, August 1, 2009

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Table 2. Major Products from Polypropylene Thermal Oxidation compound formaldehyde acetaldehyde acetone methylacrolein

product ion mass (u) 31.018 45.034 59.050 71.050

ion molecular formula +

CH3O C2H5O+ C3H7O+ C4H7O+

rate constanta,b 3.339 3.740 3.940 3.8

a Rate constant for the gas-phase reaction with H3O+, 10-9 molecule-1 · cm3 · s-1. b See the Experimental Section about the choice among the reported rate constant values.

(7) Detection of the signal (50 ms). The transient signal is acquired for 50 ms. Depending on the mass resolution needed, data treatment may cover only part of the signal (10-50 ms). (8) Ion quench (20 ms). All the ions in the cell are ejected for the next sequence. For real time following, the repetition rate of the sequence was 2.5 s. Mass Calibration. Due to its high mass accuracy, FTICR spectrometry makes identification of the molecular formulas possible. Experimental masses are obtained from the observed frequencies ν using a two-parameter calibration law m ) A/ν + B/ν2. The A and B parameters are adjusted using either ions of known molecular formula present in the mixture or ions from chemical standards introduced for calibration purposes. In the present work, mass calibration was conveniently carried out using the protonation products obtained from a standard gas containing ethanol, acetone, benzene, and toluene (50 ppm each) in air. For all identified compounds the difference between experimental and calculated masses was less than 5 × 10-3 u. Data Analysis. Each of the mass spectra contains a peak corresponding to H3O+ reactant ions, and a variable number of peaks corresponding to MiH+ ions arising from protonation of molecules Mi present in the sample. For shortest reaction times, the secondary reactions are negligible and only the primary protonation reactions are to be considered. This is ensured by adequate adjustment of the reaction time, and can be checked using the H3O+ relative abundance, which should remain larger than 70%. Providing this condition, the [Mi] concentrations in the ICR cell are easily derived from the relative abundances of H3O+ and MiH+ ions, the reaction time τ, and the rate constant ki relative to the protonation reaction: H3O+ + Mi f MiH+ + H2O The relevant ion concentrations obey the following kinetic equations:

Since the only ions initially present are H3O+, the total ion concentration in the cell remains constant and equal to its initial value [H3O+]0. Therefore, (H3O+) ) [H3O+]/[H3O+]0 is the relative amount of H3O+ ions, normalized to the sum of all the ions present. Similarly, (MiH+) is defined as the relative amount of MiH+ ions, i.e., the signal for MiH+ divided by the total ion signal. In terms of ion relative amounts, resolution of the kinetic differential equations leads to the following expressions: ln (H3O+) ) -Sτ (MiH+) ) (ki[Mi]/S)(1 - e-Sτ) These relations are combined to eliminate S, which is not necessarily known: -(ln (H3O+))(MiH+) ) ki[Mi]τ(1 - (H3O+)) + + + [Mi] ) -(In (H3O ))(MiH )/kiτ(1 - (H3O ))

Two assumptions are made to derive the concentration Ci of component Mi in the air flow from its concentration [Mi] in the ICR cell: (i) The sample composition is not modified between the carrier gas outlet and the ICR cell; i.e., the mixing ratio of each VOC Mi remains constant through the pressure reductions. (ii) The neutral gas temperature into the cell is the same as the temperature of the gas flow entering through the leak valve. This temperature is likely close to room temperature: the gas flow can be considered as cooled to room temperature at the leak valve level, and the gas admitted into the cell has no time to equilibrate with the cell walls heated to 80 °C because of continuous pumping. With these assumptions Ci ) [Mi]Pflow/Pcell, where Pflow is the pressure of the gas flow and Pcell the total sample pressure directly measured in the FTICR mass spectrometer. With Pflow ) 1.0 bar, we get the numerical relationship Ci (mol · L-1) ) (1.26 × 10-18)[[Mi] (cm-3)]/[Pcell (Torr)]. The accuracy of this quantification method was checked using the standard gas mentioned above. In particular, acetone was detected in the standard gas at the expected concentration of 50 ± 7 ppm, using the rate constant given in Table 1. For the other compounds analyzed in this work, a major source of uncertainty comes from the rate constants, since literature values for a given molecule may differ by more than 30%. However, for all the compounds present in different standardization mixtures tested under our experimental conditions, the suitable rate constants were found to be very close to the highest reported values and/ or the collisional rate constants kc. The rate constant values used in this work and given in Table 2 have been selected accordingly. The total amount Ni of a compound Mi emitted by the pyrolysis of a sample at a given time t is expressed as

d[H3O+]/dt ) -[H3O+]S where S ) ∑ki[Mi] is the sum of all ki[Mi] terms corresponding to the molecules Mi undergoing protonation by H3O+, and d[MiH+]/dt ) ki[Mi][H3O+] 6016

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Ni(t) )

∫ dn

i

)F

∫ C (t) dt t

0

i

(39) Midey, A. J.; Arnold, S. T.; Viggiano, A. A. J. Phys. Chem. A 2000, 104, 2706–2709. (40) Spanel, P.; Ji, Y.; Smith, D. Int. J. Mass Spectrom. Ion Processes 1997, 165/ 166, 25–37.

In this expression, Ci(t) (mol · L-1) is the instantaneous concentration of Mi in the gas flow and F (L · min-1) is the volume flow rate of the carrier gas. The gas flow adjustment is done upstream at room temperature, and corresponds to F ) 0.074 L · min-1. For integration the concentration Ci(t) of each product is corrected by the average background signal (corresponding to ca. 0.9 µmol · L-1). The total amount of each VOC emitted during sample degradation is directly given by the final plateau of the Ni(t) curves. Thermogravimetric Analysis. Other PP slices, of about 2 mg initial mass, were placed on the plateau of a Netzsch TG 209 microbalance. Their mass changes were determined at the specified aging temperature (±1 °C) in atmospheric air with the following program: heating from ambient temperature to aging temperature at a 50 °C · min-1 rate under a nitrogen flow of 10 mL · min-1; after 1 min at aging temperature to equilibrate the system, atmospheric air is admitted (10 mL · min-1) and the sample mass is recorded continuously versus the time of exposure. RESULTS AND DISCUSSION Nature and Time Profile of the Emissions. Several PP degradation experiments were performed using samples of different masses (1.5-7 mg) under the same conditions: oven temperature 256 °C, air flow 74 mL/min. The VOC concentrations in the outlet gas were followed by PTR-FTICR using H3O+ as the reagent ion. The chemical ionization reaction of a molecule M is: M + H3O+ f MH+ + H2O. Four major compounds were detected as their protonated ions: formaldehyde, acetaldehyde, acetone, and methylacrolein. The presence of isomers cannot be excluded in the case of methylacrolein, since the information available from the experimental data is the molecular formula of the protonated ion. The characteristics of these products are summarized in Table 2. In addition to these four products, many minor compounds were detected, each representing less than 10% of the acetone signal: C6H12 and other unsaturated hydrocarbons, acetic acid and other very minor carboxylic acids. The concentrations of the four major products were plotted against the thermal degradation time of the sample. The time elapsed from sample introduction was corrected by the transport time of gas from the sample to the mass spectrometer, as explained in the Experimental Section. An example of an emission time profile is shown in Figure 2. Several successive stages clearly appear in the figure: (i) t < 0.8 min, no VOC detected; (ii) t ) 0.8-2 min, start and increase of the emissions; (iii) t ) 2-5.5 min, steady-state emissions with constant, low VOC concentrations in the outlet gas; (iv) t ) 5.5-6.3 min, fast increase of VOC emissions (maximum slope at 5.7 min), leading to a maximum in the emitted concentrations; (v) stabilization and decrease of the VOC concentrations down to complete emission stopping. Figure 3 shows the corresponding evolution of the relative amount of each product in the VOC mixture (considering the four major products only). Namely, each concentration drawn in Figure 2 has been divided by the sum of the four product concentrations after correction from background. The time range of the graph has been restricted to the region where this operation has a sense, i.e., the time region really corresponding to VOC detection. As

Figure 2. Time profile of the VOC emissions from PP thermal degradation in atmospheric air at 256 °C for the four main products. The initial sample mass is 1.5 mg. Gray and yellow shadings indicate stages ii and iv, respectively.

Figure 3. Product distribution of the VOC emissions from PP thermal degradation in atmospheric air at 256 °C during the emission time (see the text). The initial sample mass is 1.5 mg. Yellow shading indicates stage iv.

could be expected, the time profiles are fairly noisy in the 2-5 min time range corresponding to stage iii with a low, steady emission level. The signal-to-noise ratio is improved in the 5.5-9 min time range corresponding to the concentration peak of VOC emission, and falls when the emission is ending. Interestingly, the product distribution undergoes a change during stage iv, principally concerning the relative amount of formaldehyde. While during the steady-state period the concentrations of formaldehyde and methylacrolein are close to each other and ca. 4 times less than acetone and acetaldehyde concentrations, in the next stage the formaldehyde concentration comes close to the acetone and acetaldehyde concentrations. Methylacrolein remains in a constant minor relative amount, ca. 10%. The profiles shown by Figures 2 and 3 correspond to the smallest sample analyzed (1.5 mg). The profiles obtained with heavier samples exhibit the same general patterns. However, with increasing sample mass the transition between the two stages Analytical Chemistry, Vol. 81, No. 15, August 1, 2009

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Figure 4. Time profile of the integrated amount of each of the four main emitted VOCs during PP thermal degradation in atmospheric air at 256 °C. The initial sample mass is 1.5 mg. Gray and yellow shadings indicate stages ii and iv, respectively.

Figure 5. Dependence of the amount of emitted products on the sample amount, expressed in carbon amount. The lines are linear fits of the results.

tends to become more progressive. The similarities and differences between the time profiles of the four samples are listed below. (1) The time of emission start remains fairly constant (beginning of stage ii, 0.8-0.95 min), as well as the time of sharp emission increase (stage iv, maximum slope 5.1-5.7 min). (2) While in the case of the lightest sample stage iii corresponds to a remarkably constant concentration and composition of the VOC mixture, for larger samples the formaldehyde concentration, and to a lesser extent the acetone and acetaldehyde concentrations, tends to increase progressively before the beginning of step iv. Consequently, the increase of the formaldehyde relative amount is less sharp and starts at a time between 3 and 5 min. The evolution of the time profiles toward a less clear-cut pattern for the larger samples might be due to the fact that the transition between the two emission regimes does not occur at the same time in the whole sample, which is more likely in larger samples. (3) The length of stage v increases with the sample mass. The emission end time varies from 9 to 16 min. The shape of the time profile of this stage is variable and may be irregular, while the emission composition remains very stable. For the four tested samples, the emission composition does not show any significant trend. It is summarized as follows, for steady-state and fast emission stages (relative amount range in percent, average on the different samples (av.) in percent): (formaldehyde) steady emission 10-13%, av. 11.2%; fast emission 30-33%, av. 31.7%; (acetaldehyde) steady emission 32-41%, av. 35.5%; fast emission 25-29%, av. 26.7%; (acetone) steady emission 41-48%, av. 44.8%; fast emission 31-35%, av. 33.9%; (methylacrolein) steady emission 7-10%, av. 8.5%; fast emission 7-9%, av. 7.8%. For both stages acetaldehyde, acetone, and methylacrolein remain roughly in the same ratios relative to each other. Total Amount of VOCs Emitted. The VOC amount emitted from the initial time is derived from the integration of the corresponding mixing ratio over time, knowing the gas flow in the tubing containing the sample. Figure 4 shows the corresponding time profile for the four main VOCs.

The total amount of each VOC emitted is given by integration of the signals recorded over the whole time of detectable emissions, corresponding to the final plateau of the curves in Figure 4. Representing the molecular formula of the polymer by CnH2n, with a molar mass of 14 g/mol of carbon, we know the theoretical amount of carbon in each sample: 1-5 × 10-4 mol. For each VOC the total amount emitted by a given sample is converted in carbon amount and plotted against the carbon amount in the corresponding sample. The corresponding graphs are depicted in Figure 5. This figure shows that the total carbon amount emitted as each VOC is proportional to the carbon amount in the sample. The slope of the global VOC emission curve is 0.047. This means that the sample fraction recovered as VOC emissions is 4-5%. This low yield is consistent with previous reports, mentioning for instance 2% acetone yield after complete PP oxidation.17 Weighing the sample after degradation shows a large relative mass loss, 80-90%. The “missing” material may be emitted as CO and CO2 as proposed by Bernstein et al.,25 and/ or consist in compounds of low volatility (among which is water) which are deposited or adsorbed before reaching the mass spectrometer. Comparison with Other Reported Kinetic and Analytical Studies. Successive Stages of PP Oxidative Degradation. Several kinetic studies of PP thermal oxidation mention successive stages as observed in Figure 1: after an induction period, a steady-state regime is attained. A recent study, performed on PP samples similar to those used in the present work, reports time evolution of relative mass changes and of IR spectra in the carbonyl absorption region, at three different temperatures (190, 200, and 230 °C).30 The corresponding relative mass changes and those obtained at a fourth temperature (250 °C) for the present study are reported in Figure 6. The corresponding curves show an induction period before mass loss or carbonyl infrared absorption can be detected. Very close induction times are obtained with both techniques at a given temperature. The induction time decreases with increasing temperature. After mass loss starts, the relative mass variation

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Figure 6. Mass changes of PP films in atmospheric air between 190 and 250 °C. The initial sample mass is 2 mg.

decreases steadily until at least 20% relative mass loss, suggesting a steady-state degradation of the sample. Two characteristic times, induction time ti(T) and for instance 10% mass loss time t10(T), and a maximal mass loss rate in the steady state vm(T) can be defined in those curves at temperature T. From both times, a coarse estimation of the corresponding characteristic times at 256 °C can be derived using Arrhenius extrapolation: the dependence of ln(ti) or ln(t10) on 1/T is checked to be quasi-linear, and the corresponding straight line is extrapolated to 256 °C. The resulting estimations at 256 °C are ti in the range 0.05-0.45 min with most probable value ti ≈ 0.30 min and t10 ≈ 4.4 min. In the present work, the time of the first detectable emissions is ca. 0.8 min. Therefore, the induction time corresponding to IR or mass loss measurements likely corresponds to absence of detectable VOC emission with the available sensitivity. The time before the first detectable emissions is actually longer than the predicted value of ti at the temperature used. This additional delay may correspond to the time required for heating the small quartz vessel containing the sample. The 4.4 min 10% mass loss time predicted at 256 °C belongs to the time range 2-5 min corresponding to the steady-state VOC emissions observed in the present experiments. Therefore, steadystate emissions are likely accompanied by a quasi-linear mass decrease, which makes sense. This time range also corresponds to the zone of a quasi-linear increase of the carbonyl group concentration predicted at 256 °C. The acceleration stage observed next in the PTR-FTICR studies would have been expected to appear as a sharp decrease on the relative mass variation curve. Surprisingly, it is not observed, even for the thermogravimetric analysis at 250 °C. At present this difference is difficult to explain, since it seems unlikely that accelerated VOC emission does not parallel accelerated material degradation and bond breaking and therefore accelerated mass loss. As another possibility, the differences in experimental conditions, specifically the air flow over the sample (10 mL/min for TGA, 74 mL/min for PTR-FTICR) might induce an important difference in the system kinetic behavior: in the case of the slowest air flow, the sharp degradation acceleration would be significantly delayed, so that the steady-state emissions would continue until complete sample degradation. Composition and Amount of VOC Emissions. Each of the four major products detected in the emissions appears in literature concerning VOCs from PP thermal oxidation under different

conditions, but only one report mentions all of them.24 Similarly to our results, methylacrolein is less abundant than the other product by a factor of 2-4, the other product proportions being different from those reported here. Another abundant product mentioned in this report is acetic acid, detected only as a few percent of acetone in the present study. Similarly, we detect only traces of methanol, while it amounts to more than half that of acetone in another study.17 Consistent with our findings, the volatiles represent only 2-5% of the total sample according to the reported quantitative measurements.17,23 Another way of comparing the amount emitted as VOCs to the amount lost from the material is to consider the steady-state stage. The integration time profiles of the four major VOCs considered are quasi-linear during this stage. The slope of the integration time profile relative to the total carbon amount emitted as those four VOCs was plotted versus the total carbon amount contained in the sample, and found to be roughly proportional to the latter. The corresponding proportionality constant is 9.7 × 10-4 emitted carbon per sample-contained carbon and per minute. It is referred to as the relative VOC emission rate. This value is compared to the slope of the relative mass loss curve predicted at 256 °C using Arrhenius extrapolation: 2.26 × 10-2 min-1. As a result the relative VOC emission rate is 4.3% of the relative mass loss rate during the steady-state stage. This value is consistent with the results observed after the end of degradation; i.e., the ratio of the sample fraction recovered as VOC emissions (4.7%) to sample mass loss measured in the present experiments (80-90%) is a few percent. Mechanisms for VOC Formation. The mechanism of PP thermal oxidation is considered as proceeding by a radical chain mechanism.30,32 On the basis of reported mechanistic studies of thermal oxidation of PP41-44 and similar polymers,45,46 likely mechanisms accounting for the formation of the four major VOCs detected have been proposed. Their extended discussion can be found as Supporting Information to this paper. As for its main features, this reaction scheme involves not only tertiary but also secondary and primary radicals, and proceeds mainly by intermolecular propagation between polymer chains. CONCLUSIONS The VOC emissions from PP thermal oxidation have been analyzed and quantified in real time using PTR-FTICR. The advantages of this technique over GC are easy quantification, real time analysis, and easy detection of all products simultaneously, including very volatile and/or polymerizable compounds such as formaldehyde. The emitted VOCs consist in four major products, acetone, formaldehyde, acetaldehyde, and methylacrolein, along with numerous minor compounds. Quantitative analysis indicates that only a few percent of the consumed polymer is emitted as VOCs. The time dependence of the emissions showed a complex (41) Pryor, W. A. Free Radicals; McGraw-Hill Series in Advanced Chemistry; McGraw-Hill: New York, 1969; Chapter 17, pp 280-282. (42) Chien, J. C. W.; Jabloner, H. J. Polym. Sci., Part A-1 1968, 6, 393–402. (43) Chien, J. C. W.; Boss, C. R.; Jabloner, H.; Vandenberg, E. J. J. Polym. Sci., Polym. Lett. Ed. 1972, 10, 915–919. (44) Faulkner, D. L. Polym. Eng. Sci. 1982, 22, 466–471. (45) Colin, X.; Audouin, L.; Verdu, J. Polym. Degrad. Stab. 2007, 92, 886–897. (46) Coquillat, M.; Verdu, J.; Colin, X.; Audouin, L.; Nevie`re, R. Polym. Degrad. Stab. 2007, 92, 1334–1342.

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profile consisting successively in an induction period, a stationary low emission rate, and final rapid degradation of the sample. This real time analysis technique has allowed observation of a significant change in the emission composition, occurring concurrently with the fast emission increase. This change consists in an increase of the formaldehyde relative amount, from 10% to ca. 30%, while the other products remain in constant proportions. The present results show that PTR-FTICR is an efficient tool for real time monitoring of VOC emissions from material degradation. They may be of practical interest because of the possible toxicity of the emitted compounds. In the case of polypropylene, acetaldehyde, formaldehyde, and methylacrolein need to be controlled due to their established toxicity, which may become a health concern if the material happens to be overheated in air. In addition, a high formaldehyde proportion in the emissions may be used as an indicator of fast degradation behavior of the sample. The present results may also serve as a basis for improvement of kinetic and mechanistic models for PP thermal oxidation. A planned development of material degradation study using

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PTR-FTICR consists in direct coupling between the PTR-FTICR instrument and a thermogravimetric analysis device. The resulting experiments will be very useful in the case of PP thermal oxidation: by providing an explanation to the kinetic differences observed between PTR-FTICR and thermogravimetric studies, they may contribute to better understanding of the thermal oxidation mechanism. ACKNOWLEDGMENT We thank Nicolas Bouton (AlyTech) for his help in the design of the experimental setup. SUPPORTING INFORMATION AVAILABLE Additional information as noted in text. This material is available free of charge via the Internet at http://pubs.acs.org. Received for review November 6, 2008. Accepted June 9, 2009. AC802353R