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Thermal decomposition of photooxidized isotactic polypropylene
Journal of Analytical and Applied Pyrolysis 56 (2000) 229 – 242 www.elsevier.com/locate/jaap
Thermal decomposition of photooxidized isotactic polypropylene Zs. Cze´ge´ny a,*, E. Jakab a, A. Vı´g b, B. Zelei a, M. Blazso´ a a
Research Laboratory for Materials and En6ironmental Chemistry, Chemical Research Center, Hungarian Academy of Sciences, P.O. Box 17, 1525 Budapest, Hungary b Research Group of the Hungarian Academy of Sciences, Organic Chemical Technology, P.O. Box 91, H-1521 Budapest, Hungary Received 2 February 2000; accepted 3 May 2000
Abstract Thermal decomposition of photooxidized isotactic polypropylene (iPP) samples were studied using analytical pyrolysis and thermal analysis techniques. It was found by thermogravimetry/mass spectrometry that the photooxidation decreases the thermal stability of the iPP. Oxygen-containing compounds are released over the range 100 – 300°C from the photooxidized iPP matrix. The beginning and the maximal rate of the decomposition are shifted to lower temperatures with the advancement of the oxidation of the iPP, however, the composition of the oligomer mixture formed did not change significantly. Detailed analysis of the volatile products was performed by pyrolysis-mass spectrometry and pyrolysis-gas chromatography/mass spectrometry. Volatile photo- and thermal decomposition products have been identified, some of them not yet reported. Mechanisms for their formation are proposed. © 2000 Elsevier Science B.V. All rights reserved. Keywords: Polypropylene; Pyrolysis reactions; Photooxidation products; TG/MS; Py-MS; Py-GC/MS
1. Introduction Polypropylene (PP) is one of the most extensively produced polymer, especially widely used as agricultural foils. Waist PP foils — having been exposed to sunshine for seasons — are heavily photooxidized due to the relative high photosensitivity of * Corresponding author. Tel.: +36-1-3257760/230; fax: +36-1-3257892. E-mail address: [email protected] (Z. Cze´ge´ny). 0165-2370/00/$ - see front matter © 2000 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 5 - 2 3 7 0 ( 0 0 ) 0 0 0 9 6 - 6
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PP, hence changes are expected in the thermal behavior and decomposition products. The accelerated photoaging of PP leads to oxygen-containing end-groups, macroradicals and molecular products. For the determination of non-volatile products in oxidized isotactic PP (iPP) samples IR spectrometry [1,2] and IR coupled with chemical derivatization [3] are the most widely used analytical tools. The low molecular weight oxidation products can migrate from the polymer matrix to the surface and evaporate, resulting in a not negligible weight loss during the photooxidation process. Carlsson and Wiles [1] identified carbon monoxide and acetone as major volatile products of photooxidation of PP by gas chromatograph. They proposed a mechanism for the formation of carbon monoxide by a Norrish type I scission and for that of acetone by a Norrish type II scission from polymeric ketones and methyl chain-ended ketones, respectively. Philippart et al. [4] have identified the volatile products by mass spectrometry and recently by gas chromatography [5]. They found that acetone, acetic acid, methanol, carbon monoxide, carbon dioxide and water evaporate from the pre-oxidized polymer sample under vacuum. They proposed a mechanism for the formation of acetone by double b-scission of a hydroperoxide group, or by b-scission of a methyl ketone end group. Acetic acid can be produce by the attack of a hydroxyl radical on methyl ketone. Delprat et al. [6] have analyzed the volatile products of polyphasic ethylene–propylene polymers during the photooxidation process by high performance liquid chromatography (HPLC). They have found that several peaks of the HPLC chromatogram coincided with that of model compounds such as acetic acid, acetone, a-methyl levulinic acid, g-butyrolactone and d-valerolactone. They proposed a mechanism for the formation of a-methyl levulinic acid. Girois et al. [7] described a weight loss mechanism by a zip-like backbiting reaction leading to the elimination of an oxidized dimer fragment, like a-methyl levulinic acid. The photooxidation has a marked effect on the physical properties of unstabilized PP. The polymer sheet loses elasticity and becomes brittle. The melting point shifts to lower temperatures as the exposure time increases [8]. The aim of our study was to follow and elucidate the changes of thermal stability in iPP films due to photooxidation, and to get more information on the thermal decomposition of oxygen-containing moieties using pyrolysis techniques. The expected results should broaden our knowledge about the chemical changes in iPP due to photooxidation and about the effect of these changes on the recovery of oligomers from waist foils in pyrolytic recycling processes.
2. Experimental
2.1. Materials and accelerated aging Unstabilized isotactic polypropylene (BASF, Novolen 1100N) granules were melted at 180°C for 2 min under pressure to produce 100 mm thick films.
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Polypropylene films were photooxidized in a Xenotest 150S weatherometer equipped with xenon arc-lamp (1300 W, l\ 300 nm). During irradiation the black panel temperature was controlled at 45°C and the relative humidity was 55%. Samples were irradiated for different periods of time up to 674 h.
2.2. Fourier-transform infrared spectroscopy (FT-IR) FT-IR absorption spectra of iPP films were obtained with a Perkin-Elmer 1710 FT-IR spectrometer. All spectra were averaged from 200 scans at 4 cm − 1 resolution in the 4400 – 400 cm − 1 wavenumber region and were interpreted on a linear absorbance scale.
2.3. Pyrolysis methods 2.3.1. Thermogra6imetry/mass spectrometry (TG/MS) The TG/MS instrument was built from a Perkin-Elmer TGS-2 thermobalance and a Hiden HAL 3F/PIC mass spectrometer and controlled by a computer. For the examination of the thermal stability of iPP about a 1 mg samples were placed in the platinum sample pan. The samples were heated at 20°C min − 1 up to 550°C. For studying the release of oxidized compounds up to 300°C, a larger sample (about 8 mg) was heated at 20°C min − 1 up to 300°C. A portion of the volatile products was introduced into the ion source of the mass spectrometer through a glass lined metal capillary held at 300°C. The quadrupole mass spectrometer was operated at 70 eV energy. 2.3.2. Pyrolysis-gas chromatography/mass spectrometry (Py-GC/MS) Py-GC/MS measurements were carried out in a CDS pyroprobe 120 equipped with a platinum coil and quartz sample tube, coupled to a Hewlett-Packard 5985B GC/MS instrument. A flash pyrolysis method was applied at two different temperatures. For the identification and quantification of oxygen-containing compounds 350°C pyrolysis temperature was applied. About a 1.2 mg polymer sample was pyrolyzed for 20 s. Helium carrier gas at a flow rate of 20 ml min − 1 purged the pyrolysis chamber held at 200°C. The GC separation was performed on a BP-10 fused-silica capillary column (25 m long, 0.2 mm ID, 0.25 mm film thickness, 14% cyanopropylphenyl, 85% dimethyl silicone, Chrompack), temperature programmed at 10°C min − 1 heating rate from 50 to 270°C. Pyrolysis temperature was set at 500°C for 20 s to accomplish the thermal degradation of the whole sample. The GC separation of the volatile components was performed on an Ultra-1 fused-silica capillary column (25 m long, 0.2 mm ID, 0.33 mm film thickness, crosslinked methyl silicone gum, Hewlett-Packard), temperature programmed from 50 to 300°C at 10°C min − 1. The sample mass was about 0.3 mg. The pyrolysis chamber was held at 300°C. The GC/MS interface was held at 300°C, the quadrupole mass spectrometer was operated either in EI at 70 eV or in CI mode with methane reactant gas.
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2.3.3. Pyrolysis-mass spectrometry (Py-MS) Direct inlet pyrolysis was performed in the direct inlet probe of a Hewlett-Packard 5985B GC/MS instrument. The formation of volatile products was monitored by scanning the mass range from 10 to 800 Da. About 0.1 mg of the PP sample was heated from 30 to 300°C at a rate of 20°C min − 1. The mass spectrometer was operated in EI mode at 15 eV.
3. Results and discussion
3.1. The progress of photooxidation The progress of the photooxidation was followed by FT-IR spectroscopy. Fig. 1 demonstrates the FT-IR spectra of iPP irradiated for various periods of time. The gradual increase of absorbance at carbon–oxygen bonds, bands corresponding to hydroxyl and carbonyl groups are well observed in the consecutive IR spectra. The change in the absorbance of the carbonyl bands during photooxidation of PP displays an exponential curve as expected [2], exhibiting a pseudo-induction period of about 150 h for the given conditions (shown in Fig. 2).
3.2. The influence of photooxidation on the thermal stability of iPP To examine the change in the overall thermal decomposition of photooxidized iPP a TG/MS technique was used. The mass loss (TG) and the rate of mass change (DTG) of some of the photooxidized samples and the original sample are shown in Fig. 3. The beginning and the maximal rate of the decomposition are shifted to lower temperatures with the progress of the oxidation of the iPP. The reason of the
Fig. 1. Comparison of the FT-IR spectra of photooxidised iPP films.
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Fig. 2. Change in the carbonyls absorption during photooxidation of PP films.
Fig. 3. TG and DTG curves of photooxidized iPP samples. Solid line, original sample; dotted line, 168 h irradiated sample; dashed line, 354 h irradiated sample; dashed-dotted line, 534 h irradiated sample.
shifts to lower decomposition temperatures with increased exposure time is the formation of oxidized moieties of lower thermal stability, moreover, unsaturated groups are also produced by polymer chain scission during the photodegradation process also causing reduced thermal stability. The temperatures of DTG maxima and the corresponding DTG values have been plotted against time of photooxidation in Figs. 4 and 5, respectively. Both plots display a pseudo-induction period similar to that observed in the formation of carbonyl IR absorption bands. The photooxidation does not have a measurable influence on the char yield of iPP. It was more than 1.2% and less than 2.8% for all the samples, calculated from the final mass loss of TG curves above 500°C. Fig. 6 presents the evolution rates of selected products and the DTG curve of the most photooxidized iPP sample up to 300°C. The DTG curve shows two main maxima of mass loss rate in this temperature range. The first maximal rate is reached at the melting point of the oxidized polymer sample (at 140°C) due to the
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evaporation of the volatile compounds formed during the photooxidation process, but remaining enclosed in the inclusions in the solid polymer matrix. With further heating the melt from 150 to 300°C oxygen-containing compounds are evolved by thermal decomposition of the oxygen-containing functional groups. The evolution rates of selected products show that the main oxygen-containing compounds (water, carbon-dioxide, acetone) are released between 100 and 300°C. The mass loss measured up to 300°C amounts to 4.6% of the most photooxidized sample while it is only 0.8% of the original iPP sample. The decomposition of the residual iPP to hydrocarbons starts at about 280°C.
Fig. 4. Variation of the temperature of the DTG maximum with the exposure time of photooxidation of iPP samples.
Fig. 5. Variation of the maximum value of the DTG curve with the exposure time of photooxidation of iPP.
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Fig. 6. TG/MS curves of the most photooxidised iPP sample (exposed for 674 h). solid lines, TG and DTG; dashed line, water (m/z= 18); dotted line, carbon-dioxide (m/z = 44); and dashed-dotted line, acetone (m/z= 58).
3.3. Analysis of oxygen-containing products The identification and quantification of the pyrolysis products were carried out by Py-GC/MS. As is well known [9,10] polypropylene decomposes mainly by the evolution of oligomeric products. Fig. 7 illustrates the low temperature pyrolysisgas chromatograms of the original (a) and the most photooxidized iPP sample (b). The TG/MS curves demonstrated that the evolution of oxygen-containing products occurs at low temperature, therefore, a 350°C pyrolysis temperature was applied. The comparison of chromatograms shows that the peaks of hydrocarbon components (oligomers) are considerably increased due to photooxidation, moreover, numerous new compounds are formed. The new products were identified by interpreting CI-MS spectra and by matching EI-MS spectra with library spectra. In addition to carbon dioxide, acetic acid and acetone (already published in the literature), we could identify some other minor volatile oxygen-containing products, indicated in Fig. 7, as 2,4-dimethyl-furan, 2,4,-pentanedione, 4-hydroxi-4-methyl-2pentanone and 3,5-dimethyl-5-isobutyl-2,5-dihydrofuran-2-one. Their spectra are shown in Fig. 8. There are three peaks of hydroxy-ketone compounds in the chromatogram exhibiting the same typical fragments in their MS spectra. The peak of the shortest retention was identified as 4-hydroxy-4-methyl-2-pentanone. However, we could not detect a-methyl levulinic acid, g-butyrolactone and d-valerolactone among the thermal degradation products. Peak area values of the molecular ions (or that of an abundant characteristic fragment ion) normalized to the mass of polymer sample have been plotted against the time of photooxidation in Fig. 9. For the quantification of oxygen containing products peak area of a single ion was used due to the poor resolution of the peaks.
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Only relative yields were evaluated on the basis of single ion peak areas. The amount of carbon dioxide and acetic acid increases continuously with the time of accelerated aging. The relatively high yield of carbon dioxide is in accordance with the observation that the main photooxidation route leads to the formation of carboxylic acids [11]. Although the reproducibility of the peak areas of the minor products was rather poor, the trend of the increasing yield is noticeable. In the case of the ketonic compounds (acetone; 2,4,-pentanedione; 4-hydroxy-4-methyl-2-pentanone) there is a maximum in the yield. The possible reason for the drop in yield at longer irradiation times could be that a part of the ketonic products are further oxidized, resulting in carboxyl compounds. For monitoring the release of oxygen-containing compounds under pyrolysis, direct inlet pyrolysis was used. The shape of the total ion curve of volatiles produced from the most photooxidized polymer sample shown in Fig. 10, is in accordance with the results above obtained by TG-MS, namely, the volatile compounds leave the iPP matrix in two steps. The first maximum of the total ion curve at the melting point of the photooxidized iPP (at 140°C) arises from the evaporation of the volatile compounds from the inclusions, and the second one (at 270°C) originates from the volatile product evolution by thermal decomposition. The ion curves show that each of oxygenated compounds are evolved in both steps except carbon dioxide. The main part of acetic acid forms by thermal decomposi-
Fig. 7. Pyrolysis-GC/MS (350°C) total ion chromatograms. (a) Original iPP sample and (b) most photooxidised iPP sample
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Fig. 8. EI MS and CI MS spectra of the identified oxygen-containing pyrolysis products.
tion, acetone is evolved in both steps. Carbon dioxide detected above 230°C must be formed by decarboxylation of the oxidized polymer chain. The oxygen containing compounds with higher molecular mass are mostly formed during the photodegradation process and released from the sample at the melting point.
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Fig. 9. Yields of oxygen-containing products from photooxidised iPP samples at 350°C pyrolysis temperature. (a) --, Carbon dioxide (m/z =44); - -, acetone (m/z = 56); - -, acetic acid (m/z= 60). (b) --,2,4-Dimethyl-furan (m/z =96); --,2,4-pentanedione (m/z= 100); - -,4-hydroxy-4-methyl-2pentanone (m/z = 59); - -,3,5-dimethyl-5-isobutyl-2,5-dihydrofuran-2-one (m/z =111).
Fig. 10. Ion curves of direct inlet pyrolysis products. (a) Solid line, total ion curve; dotted line, water (m/z= 18); dashed-dotted-dotted line, carbon dioxide (m/z= 44); dashed-dotted line, acetone (m/z= 58); dashed line, acetic acid (m/z =60). (b) Solid line, total ion curve; dashed-dotted-dotted line, 4-hydroxy-4-methyl-2-pentanone and hydroxy-ketones (m/z =101); dashed line, 2,4-dimethyl-furan (m/ z= 96); dashed-dotted line, 2,4-pentanedione (m/z= 100); dotted line, 3,5-dimethyl-5-isobuthyl-2,5-dihydrofuran-2-one (m/z =168).
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Scheme 1. Formation of identified compounds from methyl ketone chain-end groups.
Scheme 2. Origin of 2,4-dimethyl furan.
3.4. The origin of oxygen-containing products The formation of carbon dioxide, acetone and acetic acid could be interpreted by the photodecomposition reactions described in the literature [2]. These compounds may be released from the inclusions or formed by thermal decomposition reactions in similar reactions to photodecomposition. In order to understand the formation of the oxygen-containing compounds identified in this work reaction paths are proposed in Schemes 1 – 3, for the thermal decomposition of photooxidized iPP chain-end groups described in the literature [2,5]. In oxidized iPP there are tertiary hydroperoxides associated in sequences [12,13]. Their decomposition leads to methyl ketone [14] chain-end groups, with tertiary hydroperoxides in positions 3,5… (Scheme 1). Both thermal and photo-exposure can cause the initial scission of the weak OO link of hydroperoxide. The free-radical product may abstract a hydrogen atom to produce tertiary alcohol. The
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alkoxyl radical produced by scission of the OO link of hydroperoxide in position 5 could form 4-hydroxy-4-methyl-2-pentanone and a methyl ketone chain-end group by chain-scission and subsequent hydrogen abstraction. Another reaction way of the free radical formed by scission of the OO bond of hydroperoxide in position 3 is the chain-scission in the b position to produce 2,4-pentanedione as a stable compound. Among the oxygen-containing products, methyl-ketone conjugated alkene chainend groups also form during the photooxidation [15]. Aromatization following the chain scission in the allyl position (proposed in Scheme 2, lower reaction) results in a very stable compound: 2,4-dimethyl-furan. The tertiary hydroperoxide substituted methyl ketone conjugated alkene chain-end group could also form 2,4-dimethyl-furan by OO scission and b-scission ( Scheme 2, upper reaction). A glactone chain-end group was identified by IR spectroscopy [15]. The homolytic scission of the g-lactone end group leads to the formation of a secondary and a primary radical (Scheme 3). An intramolecular radical transfer from the primary radical to a tertiary carbon atom should take place. From this radical 3,5-dimetyl-5-isobutyl-2,5-dihydrofuran-2-one may form by hydrogen elimination to produce conjugated double bonds.
3.5. Quantitati6e analysis of the pyrolysis oil of iPP Although the beginning of the thermal decomposition shifts to a lower temperature in the irradiated sample, the composition of the oligomer mixture formed at 500°C pyrolysis temperature (pyrolysis oil) does not change significantly in comparison to the original iPP (Table 1), indicating a similar decomposition mechanism [16]. The markedly increased hydrocarbon (oligomer) yield at 350°C pyrolysis temperature from photooxidized PP (Fig. 7) is due to macroradicals formed by the cleavage of hydroperoxy groups or allyl CC bonds which could occur at considerably lower temperatures than the thermal scission of CC bonds of the polymer chain.
Scheme 3. Formation of 3,5-dimethyl-5-izobuthyl-2,5-dihydrofuran-2-one.
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Table 1 Composition of the monomer–pentamer fraction of iPP pyrolysis oil (values in % peak area)
C3 hydrocarbonsa C5 hydrocarbonsa C6 hydrocarbonsa Trimer Tetramersb Petramersb a b
Original iPP foil
Most photooxidised iPP foil
12.19 0.9 14.090.7 8.8 91.3 36.891.9 7.89 0.7 20.592.5
11.5 9 0.9 12.6 9 1.0 8.3 90.2 39.4 9 2.1 7.5 90.9 20.6 9 1.7
Unresolved peaks. Sum of the isomers peak area.
4. Conclusions The results can be summarized and interpreted as follows: Oxygen-containing compounds are released from the photooxidized iPP in two steps between 100 – 300°C. One part of the low molecular mass products formed by photooxidation remain in the polymer matrix and is capable of leaving it only at the melting point of the oxidized iPP. Between 170–300°C low molecular mass oxygen-containing compounds are formed by thermal degradation of the functional groups of the oxidized polymer sample. The evolution of some oxygen-containing minor degradation products are related to the oxygen-containing chain-end groups formed under photooxidation. The decomposition maximum of iPP shift to lower temperatures as the exposure time is increased, because of the formation of radicals at lower temperatures from the photooxidized moieties of the polymer. There is no notable change due to irradiation in the composition of the oligomeric thermal decomposition products at a higher pyrolysis temperature (500°C). Those parts of the iPP macromolecules which are attacked by weathering are released at considerably lower temperatures than the recovery of oligomers in a pyrolytic recycling process.
Acknowledgements This work was supported by the Hungarian National Research Fund (OTKA contract No. T 022091), and by the EC (INCO-Copernicus project No. IC15-CT960717).
References [1] D.J. Carlsson, D.M. Wiles, Macromolecules 2 (1969) 587. [2] J.H. Adams, J. Polym. Sci. A 8 (1970) 1077.
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[3] J. Lacoste, D. Vaillant, D.J. Carlsson, J. Polym. Sci. A 31 (1993) 715. [4] J.-L. Philippart, F. Posada, J.-L. Gardette, Polym. Degrad. Stab. 49 (1995) 285. [5] S. Commereuc, D. Vaillant, J.-L. Philippart, J. Lacoste, D.J. Carlsson, Polym. Degrad. Stab. 57 (1997) 175. [6] P. Delprat, X. Duteurtre, J.-L. Gardette, Polym. Degrad. Stab. 50 (1995) 1 – 12. [7] S. Girois, L. Audouin, P. Delprat, J. Verdu, Polym. Degrad. Stab. 51 (1996) 133. [8] M.S. Rabello, J.R. White, Polymer 38 (26) (1997) 6379. [9] Y. Tsuchiya, K. Sumi, J. Polym. Sci. A-1 7 (1969) 1599. [10] R.P. Lattimer, J. Anal. Appl. Pyrolysis 31 (1995) 203. [11] J.-L. Gardette, C. Sinturel, J. Lemaire, Polym. Degrad. Stab. 64 (1999) 411. [12] D.J. Carlsson, D.M. Wiles, J. Macromol, Sci.-Rev. Macromol. Chem. C14 (1) (1976) 65. [13] C.W. Chien, E.J. Vandenberg, H. Jabloner, J. Polym. Sci. A-1 6 (1968) 381. [14] D.J. Carlsson, D.M. Wiles, Macromolecules 2 (1969) 597. [15] J.H. Adams, J. Polym. Sci. A 8 (1970) 1279. [16] J.K.Y. Kiang, P.C. Uden, J.C.W. Chien, Polym. Degrad. Stab. 2 (1980) 113.
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Thermal decomposition of photooxidized isotactic polypropylene Zs. Cze´ge´ny a,*, E. Jakab a, A. Vı´g b, B. Zelei a, M. Blazso´ a a
Research Laboratory for Materials and En6ironmental Chemistry, Chemical Research Center, Hungarian Academy of Sciences, P.O. Box 17, 1525 Budapest, Hungary b Research Group of the Hungarian Academy of Sciences, Organic Chemical Technology, P.O. Box 91, H-1521 Budapest, Hungary Received 2 February 2000; accepted 3 May 2000
Abstract Thermal decomposition of photooxidized isotactic polypropylene (iPP) samples were studied using analytical pyrolysis and thermal analysis techniques. It was found by thermogravimetry/mass spectrometry that the photooxidation decreases the thermal stability of the iPP. Oxygen-containing compounds are released over the range 100 – 300°C from the photooxidized iPP matrix. The beginning and the maximal rate of the decomposition are shifted to lower temperatures with the advancement of the oxidation of the iPP, however, the composition of the oligomer mixture formed did not change significantly. Detailed analysis of the volatile products was performed by pyrolysis-mass spectrometry and pyrolysis-gas chromatography/mass spectrometry. Volatile photo- and thermal decomposition products have been identified, some of them not yet reported. Mechanisms for their formation are proposed. © 2000 Elsevier Science B.V. All rights reserved. Keywords: Polypropylene; Pyrolysis reactions; Photooxidation products; TG/MS; Py-MS; Py-GC/MS
1. Introduction Polypropylene (PP) is one of the most extensively produced polymer, especially widely used as agricultural foils. Waist PP foils — having been exposed to sunshine for seasons — are heavily photooxidized due to the relative high photosensitivity of * Corresponding author. Tel.: +36-1-3257760/230; fax: +36-1-3257892. E-mail address: [email protected] (Z. Cze´ge´ny). 0165-2370/00/$ - see front matter © 2000 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 5 - 2 3 7 0 ( 0 0 ) 0 0 0 9 6 - 6
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PP, hence changes are expected in the thermal behavior and decomposition products. The accelerated photoaging of PP leads to oxygen-containing end-groups, macroradicals and molecular products. For the determination of non-volatile products in oxidized isotactic PP (iPP) samples IR spectrometry [1,2] and IR coupled with chemical derivatization [3] are the most widely used analytical tools. The low molecular weight oxidation products can migrate from the polymer matrix to the surface and evaporate, resulting in a not negligible weight loss during the photooxidation process. Carlsson and Wiles [1] identified carbon monoxide and acetone as major volatile products of photooxidation of PP by gas chromatograph. They proposed a mechanism for the formation of carbon monoxide by a Norrish type I scission and for that of acetone by a Norrish type II scission from polymeric ketones and methyl chain-ended ketones, respectively. Philippart et al. [4] have identified the volatile products by mass spectrometry and recently by gas chromatography [5]. They found that acetone, acetic acid, methanol, carbon monoxide, carbon dioxide and water evaporate from the pre-oxidized polymer sample under vacuum. They proposed a mechanism for the formation of acetone by double b-scission of a hydroperoxide group, or by b-scission of a methyl ketone end group. Acetic acid can be produce by the attack of a hydroxyl radical on methyl ketone. Delprat et al. [6] have analyzed the volatile products of polyphasic ethylene–propylene polymers during the photooxidation process by high performance liquid chromatography (HPLC). They have found that several peaks of the HPLC chromatogram coincided with that of model compounds such as acetic acid, acetone, a-methyl levulinic acid, g-butyrolactone and d-valerolactone. They proposed a mechanism for the formation of a-methyl levulinic acid. Girois et al. [7] described a weight loss mechanism by a zip-like backbiting reaction leading to the elimination of an oxidized dimer fragment, like a-methyl levulinic acid. The photooxidation has a marked effect on the physical properties of unstabilized PP. The polymer sheet loses elasticity and becomes brittle. The melting point shifts to lower temperatures as the exposure time increases [8]. The aim of our study was to follow and elucidate the changes of thermal stability in iPP films due to photooxidation, and to get more information on the thermal decomposition of oxygen-containing moieties using pyrolysis techniques. The expected results should broaden our knowledge about the chemical changes in iPP due to photooxidation and about the effect of these changes on the recovery of oligomers from waist foils in pyrolytic recycling processes.
2. Experimental
2.1. Materials and accelerated aging Unstabilized isotactic polypropylene (BASF, Novolen 1100N) granules were melted at 180°C for 2 min under pressure to produce 100 mm thick films.
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Polypropylene films were photooxidized in a Xenotest 150S weatherometer equipped with xenon arc-lamp (1300 W, l\ 300 nm). During irradiation the black panel temperature was controlled at 45°C and the relative humidity was 55%. Samples were irradiated for different periods of time up to 674 h.
2.2. Fourier-transform infrared spectroscopy (FT-IR) FT-IR absorption spectra of iPP films were obtained with a Perkin-Elmer 1710 FT-IR spectrometer. All spectra were averaged from 200 scans at 4 cm − 1 resolution in the 4400 – 400 cm − 1 wavenumber region and were interpreted on a linear absorbance scale.
2.3. Pyrolysis methods 2.3.1. Thermogra6imetry/mass spectrometry (TG/MS) The TG/MS instrument was built from a Perkin-Elmer TGS-2 thermobalance and a Hiden HAL 3F/PIC mass spectrometer and controlled by a computer. For the examination of the thermal stability of iPP about a 1 mg samples were placed in the platinum sample pan. The samples were heated at 20°C min − 1 up to 550°C. For studying the release of oxidized compounds up to 300°C, a larger sample (about 8 mg) was heated at 20°C min − 1 up to 300°C. A portion of the volatile products was introduced into the ion source of the mass spectrometer through a glass lined metal capillary held at 300°C. The quadrupole mass spectrometer was operated at 70 eV energy. 2.3.2. Pyrolysis-gas chromatography/mass spectrometry (Py-GC/MS) Py-GC/MS measurements were carried out in a CDS pyroprobe 120 equipped with a platinum coil and quartz sample tube, coupled to a Hewlett-Packard 5985B GC/MS instrument. A flash pyrolysis method was applied at two different temperatures. For the identification and quantification of oxygen-containing compounds 350°C pyrolysis temperature was applied. About a 1.2 mg polymer sample was pyrolyzed for 20 s. Helium carrier gas at a flow rate of 20 ml min − 1 purged the pyrolysis chamber held at 200°C. The GC separation was performed on a BP-10 fused-silica capillary column (25 m long, 0.2 mm ID, 0.25 mm film thickness, 14% cyanopropylphenyl, 85% dimethyl silicone, Chrompack), temperature programmed at 10°C min − 1 heating rate from 50 to 270°C. Pyrolysis temperature was set at 500°C for 20 s to accomplish the thermal degradation of the whole sample. The GC separation of the volatile components was performed on an Ultra-1 fused-silica capillary column (25 m long, 0.2 mm ID, 0.33 mm film thickness, crosslinked methyl silicone gum, Hewlett-Packard), temperature programmed from 50 to 300°C at 10°C min − 1. The sample mass was about 0.3 mg. The pyrolysis chamber was held at 300°C. The GC/MS interface was held at 300°C, the quadrupole mass spectrometer was operated either in EI at 70 eV or in CI mode with methane reactant gas.
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2.3.3. Pyrolysis-mass spectrometry (Py-MS) Direct inlet pyrolysis was performed in the direct inlet probe of a Hewlett-Packard 5985B GC/MS instrument. The formation of volatile products was monitored by scanning the mass range from 10 to 800 Da. About 0.1 mg of the PP sample was heated from 30 to 300°C at a rate of 20°C min − 1. The mass spectrometer was operated in EI mode at 15 eV.
3. Results and discussion
3.1. The progress of photooxidation The progress of the photooxidation was followed by FT-IR spectroscopy. Fig. 1 demonstrates the FT-IR spectra of iPP irradiated for various periods of time. The gradual increase of absorbance at carbon–oxygen bonds, bands corresponding to hydroxyl and carbonyl groups are well observed in the consecutive IR spectra. The change in the absorbance of the carbonyl bands during photooxidation of PP displays an exponential curve as expected [2], exhibiting a pseudo-induction period of about 150 h for the given conditions (shown in Fig. 2).
3.2. The influence of photooxidation on the thermal stability of iPP To examine the change in the overall thermal decomposition of photooxidized iPP a TG/MS technique was used. The mass loss (TG) and the rate of mass change (DTG) of some of the photooxidized samples and the original sample are shown in Fig. 3. The beginning and the maximal rate of the decomposition are shifted to lower temperatures with the progress of the oxidation of the iPP. The reason of the
Fig. 1. Comparison of the FT-IR spectra of photooxidised iPP films.
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Fig. 2. Change in the carbonyls absorption during photooxidation of PP films.
Fig. 3. TG and DTG curves of photooxidized iPP samples. Solid line, original sample; dotted line, 168 h irradiated sample; dashed line, 354 h irradiated sample; dashed-dotted line, 534 h irradiated sample.
shifts to lower decomposition temperatures with increased exposure time is the formation of oxidized moieties of lower thermal stability, moreover, unsaturated groups are also produced by polymer chain scission during the photodegradation process also causing reduced thermal stability. The temperatures of DTG maxima and the corresponding DTG values have been plotted against time of photooxidation in Figs. 4 and 5, respectively. Both plots display a pseudo-induction period similar to that observed in the formation of carbonyl IR absorption bands. The photooxidation does not have a measurable influence on the char yield of iPP. It was more than 1.2% and less than 2.8% for all the samples, calculated from the final mass loss of TG curves above 500°C. Fig. 6 presents the evolution rates of selected products and the DTG curve of the most photooxidized iPP sample up to 300°C. The DTG curve shows two main maxima of mass loss rate in this temperature range. The first maximal rate is reached at the melting point of the oxidized polymer sample (at 140°C) due to the
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evaporation of the volatile compounds formed during the photooxidation process, but remaining enclosed in the inclusions in the solid polymer matrix. With further heating the melt from 150 to 300°C oxygen-containing compounds are evolved by thermal decomposition of the oxygen-containing functional groups. The evolution rates of selected products show that the main oxygen-containing compounds (water, carbon-dioxide, acetone) are released between 100 and 300°C. The mass loss measured up to 300°C amounts to 4.6% of the most photooxidized sample while it is only 0.8% of the original iPP sample. The decomposition of the residual iPP to hydrocarbons starts at about 280°C.
Fig. 4. Variation of the temperature of the DTG maximum with the exposure time of photooxidation of iPP samples.
Fig. 5. Variation of the maximum value of the DTG curve with the exposure time of photooxidation of iPP.
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Fig. 6. TG/MS curves of the most photooxidised iPP sample (exposed for 674 h). solid lines, TG and DTG; dashed line, water (m/z= 18); dotted line, carbon-dioxide (m/z = 44); and dashed-dotted line, acetone (m/z= 58).
3.3. Analysis of oxygen-containing products The identification and quantification of the pyrolysis products were carried out by Py-GC/MS. As is well known [9,10] polypropylene decomposes mainly by the evolution of oligomeric products. Fig. 7 illustrates the low temperature pyrolysisgas chromatograms of the original (a) and the most photooxidized iPP sample (b). The TG/MS curves demonstrated that the evolution of oxygen-containing products occurs at low temperature, therefore, a 350°C pyrolysis temperature was applied. The comparison of chromatograms shows that the peaks of hydrocarbon components (oligomers) are considerably increased due to photooxidation, moreover, numerous new compounds are formed. The new products were identified by interpreting CI-MS spectra and by matching EI-MS spectra with library spectra. In addition to carbon dioxide, acetic acid and acetone (already published in the literature), we could identify some other minor volatile oxygen-containing products, indicated in Fig. 7, as 2,4-dimethyl-furan, 2,4,-pentanedione, 4-hydroxi-4-methyl-2pentanone and 3,5-dimethyl-5-isobutyl-2,5-dihydrofuran-2-one. Their spectra are shown in Fig. 8. There are three peaks of hydroxy-ketone compounds in the chromatogram exhibiting the same typical fragments in their MS spectra. The peak of the shortest retention was identified as 4-hydroxy-4-methyl-2-pentanone. However, we could not detect a-methyl levulinic acid, g-butyrolactone and d-valerolactone among the thermal degradation products. Peak area values of the molecular ions (or that of an abundant characteristic fragment ion) normalized to the mass of polymer sample have been plotted against the time of photooxidation in Fig. 9. For the quantification of oxygen containing products peak area of a single ion was used due to the poor resolution of the peaks.
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Only relative yields were evaluated on the basis of single ion peak areas. The amount of carbon dioxide and acetic acid increases continuously with the time of accelerated aging. The relatively high yield of carbon dioxide is in accordance with the observation that the main photooxidation route leads to the formation of carboxylic acids [11]. Although the reproducibility of the peak areas of the minor products was rather poor, the trend of the increasing yield is noticeable. In the case of the ketonic compounds (acetone; 2,4,-pentanedione; 4-hydroxy-4-methyl-2-pentanone) there is a maximum in the yield. The possible reason for the drop in yield at longer irradiation times could be that a part of the ketonic products are further oxidized, resulting in carboxyl compounds. For monitoring the release of oxygen-containing compounds under pyrolysis, direct inlet pyrolysis was used. The shape of the total ion curve of volatiles produced from the most photooxidized polymer sample shown in Fig. 10, is in accordance with the results above obtained by TG-MS, namely, the volatile compounds leave the iPP matrix in two steps. The first maximum of the total ion curve at the melting point of the photooxidized iPP (at 140°C) arises from the evaporation of the volatile compounds from the inclusions, and the second one (at 270°C) originates from the volatile product evolution by thermal decomposition. The ion curves show that each of oxygenated compounds are evolved in both steps except carbon dioxide. The main part of acetic acid forms by thermal decomposi-
Fig. 7. Pyrolysis-GC/MS (350°C) total ion chromatograms. (a) Original iPP sample and (b) most photooxidised iPP sample
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Fig. 8. EI MS and CI MS spectra of the identified oxygen-containing pyrolysis products.
tion, acetone is evolved in both steps. Carbon dioxide detected above 230°C must be formed by decarboxylation of the oxidized polymer chain. The oxygen containing compounds with higher molecular mass are mostly formed during the photodegradation process and released from the sample at the melting point.
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Fig. 9. Yields of oxygen-containing products from photooxidised iPP samples at 350°C pyrolysis temperature. (a) --, Carbon dioxide (m/z =44); - -, acetone (m/z = 56); - -, acetic acid (m/z= 60). (b) --,2,4-Dimethyl-furan (m/z =96); --,2,4-pentanedione (m/z= 100); - -,4-hydroxy-4-methyl-2pentanone (m/z = 59); - -,3,5-dimethyl-5-isobutyl-2,5-dihydrofuran-2-one (m/z =111).
Fig. 10. Ion curves of direct inlet pyrolysis products. (a) Solid line, total ion curve; dotted line, water (m/z= 18); dashed-dotted-dotted line, carbon dioxide (m/z= 44); dashed-dotted line, acetone (m/z= 58); dashed line, acetic acid (m/z =60). (b) Solid line, total ion curve; dashed-dotted-dotted line, 4-hydroxy-4-methyl-2-pentanone and hydroxy-ketones (m/z =101); dashed line, 2,4-dimethyl-furan (m/ z= 96); dashed-dotted line, 2,4-pentanedione (m/z= 100); dotted line, 3,5-dimethyl-5-isobuthyl-2,5-dihydrofuran-2-one (m/z =168).
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Scheme 1. Formation of identified compounds from methyl ketone chain-end groups.
Scheme 2. Origin of 2,4-dimethyl furan.
3.4. The origin of oxygen-containing products The formation of carbon dioxide, acetone and acetic acid could be interpreted by the photodecomposition reactions described in the literature [2]. These compounds may be released from the inclusions or formed by thermal decomposition reactions in similar reactions to photodecomposition. In order to understand the formation of the oxygen-containing compounds identified in this work reaction paths are proposed in Schemes 1 – 3, for the thermal decomposition of photooxidized iPP chain-end groups described in the literature [2,5]. In oxidized iPP there are tertiary hydroperoxides associated in sequences [12,13]. Their decomposition leads to methyl ketone [14] chain-end groups, with tertiary hydroperoxides in positions 3,5… (Scheme 1). Both thermal and photo-exposure can cause the initial scission of the weak OO link of hydroperoxide. The free-radical product may abstract a hydrogen atom to produce tertiary alcohol. The
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alkoxyl radical produced by scission of the OO link of hydroperoxide in position 5 could form 4-hydroxy-4-methyl-2-pentanone and a methyl ketone chain-end group by chain-scission and subsequent hydrogen abstraction. Another reaction way of the free radical formed by scission of the OO bond of hydroperoxide in position 3 is the chain-scission in the b position to produce 2,4-pentanedione as a stable compound. Among the oxygen-containing products, methyl-ketone conjugated alkene chainend groups also form during the photooxidation [15]. Aromatization following the chain scission in the allyl position (proposed in Scheme 2, lower reaction) results in a very stable compound: 2,4-dimethyl-furan. The tertiary hydroperoxide substituted methyl ketone conjugated alkene chain-end group could also form 2,4-dimethyl-furan by OO scission and b-scission ( Scheme 2, upper reaction). A glactone chain-end group was identified by IR spectroscopy [15]. The homolytic scission of the g-lactone end group leads to the formation of a secondary and a primary radical (Scheme 3). An intramolecular radical transfer from the primary radical to a tertiary carbon atom should take place. From this radical 3,5-dimetyl-5-isobutyl-2,5-dihydrofuran-2-one may form by hydrogen elimination to produce conjugated double bonds.
3.5. Quantitati6e analysis of the pyrolysis oil of iPP Although the beginning of the thermal decomposition shifts to a lower temperature in the irradiated sample, the composition of the oligomer mixture formed at 500°C pyrolysis temperature (pyrolysis oil) does not change significantly in comparison to the original iPP (Table 1), indicating a similar decomposition mechanism [16]. The markedly increased hydrocarbon (oligomer) yield at 350°C pyrolysis temperature from photooxidized PP (Fig. 7) is due to macroradicals formed by the cleavage of hydroperoxy groups or allyl CC bonds which could occur at considerably lower temperatures than the thermal scission of CC bonds of the polymer chain.
Scheme 3. Formation of 3,5-dimethyl-5-izobuthyl-2,5-dihydrofuran-2-one.
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Table 1 Composition of the monomer–pentamer fraction of iPP pyrolysis oil (values in % peak area)
C3 hydrocarbonsa C5 hydrocarbonsa C6 hydrocarbonsa Trimer Tetramersb Petramersb a b
Original iPP foil
Most photooxidised iPP foil
12.19 0.9 14.090.7 8.8 91.3 36.891.9 7.89 0.7 20.592.5
11.5 9 0.9 12.6 9 1.0 8.3 90.2 39.4 9 2.1 7.5 90.9 20.6 9 1.7
Unresolved peaks. Sum of the isomers peak area.
4. Conclusions The results can be summarized and interpreted as follows: Oxygen-containing compounds are released from the photooxidized iPP in two steps between 100 – 300°C. One part of the low molecular mass products formed by photooxidation remain in the polymer matrix and is capable of leaving it only at the melting point of the oxidized iPP. Between 170–300°C low molecular mass oxygen-containing compounds are formed by thermal degradation of the functional groups of the oxidized polymer sample. The evolution of some oxygen-containing minor degradation products are related to the oxygen-containing chain-end groups formed under photooxidation. The decomposition maximum of iPP shift to lower temperatures as the exposure time is increased, because of the formation of radicals at lower temperatures from the photooxidized moieties of the polymer. There is no notable change due to irradiation in the composition of the oligomeric thermal decomposition products at a higher pyrolysis temperature (500°C). Those parts of the iPP macromolecules which are attacked by weathering are released at considerably lower temperatures than the recovery of oligomers in a pyrolytic recycling process.
Acknowledgements This work was supported by the Hungarian National Research Fund (OTKA contract No. T 022091), and by the EC (INCO-Copernicus project No. IC15-CT960717).
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