La importancia de la apariencia física

E. Jakab^1*, T. Bhaskar² and Y. Sakata²

1Institute of Materials and Environmental Chemistry, Chemical Research Center, Hungarian Academy of Sciences, H-1525 Budapest P.O. Box 17, Hungary

2Department of Applied Chemistry, Okayama University, 3-1-1 Tsushima-Naka, Okayama 700-8530, Japan, [email protected]

Abstract: The thermal decomposition of various mixtures of ABS, PET and PVC has been studied in order to clarify the reactions between the components of mixed polymers. ABS contains brominated epoxi resin fl ame retardant and antimony trioxide synergist. It was established that the decomposition rate curves (DTG) of the mixtures were different from the summed curves of the individual components indicating the interactions between the decomposition reactions of the polymer components. The dehydrochlorination rate of PVC was sensitive for the presence of other components. Brominated and chlorinated aromatic esters were detected from the mixtures containing PET and halogen-contain-ing polymers. It is concluded that reactions occur between the polymer components and fl ame retardants and hydrogen chloride released from PVC.

1. Introduction

Incineration of containing polymers leads to the formation of harmful halogen-ated dioxins. Therefore pyrolysis seems a more promising alternative for the recycling of halogen-containing plastic waste. Pyrolysis produces valuable chemicals or fuels from waste plastics. However, pyrolytic recycling of waste plastics of halogen content requires special attention since it should be combined with the elimination of halogenated prod-ucts. In order to elaborate a proper dehalogenation process, detailed knowledge of the reaction mechanisms of the formation of halogenated products is necessary. Luda et al [1] studied the thermal decomposition of fi re retardant epoxi resins based on diglycidyl ether of bisphenol A and identifi ed various mono- and dibrominated phenols and aliphatic products. The formation of various brominated aromatic products was monitored dur-ing pyrolysis of fl ame-retarded high-impact polystyrene (HIPS) [2]. Chlorinated organic compounds have been identifi ed in the pyrolysis oil of the mixture of poly(vinyl chloride) (PVC) and poly(ethylene terephthalate) (PET) [3]. PVC generally affects the decomposi-tion of other polymers due to the catalytic effect of HCl released [4-7]. Dehydrochlorina-tion of PVC is promoted by the presence of polyamides and polyacrylonitrile [8]. Sakata et al. [9-11] studied the catalytic dehalogenation process of chlorinated and brominated or-ganic compounds formed during the pyrolysis of mixed plastics. Knümann and Bochorn [12] have studied the decomposition of PVC-containing polymer mixtures and concluded

that a separation of plastic mixtures is possible by temperature-controlled pyrolysis in recycling processes.

The major components of the waste from electric and electronic equipment are HIPS and acrylonitrile-butadiene-styrene copolymer (ABS), which contain brominated fl ame re-tardants. However, the waste plastics from residential electronics recycling contain other polymers including PE, PP, PVC and PET. The components of mixed plastics infl uence the thermal decomposition of each others during pyrolysis. The aim of the present work is to investigate the thermal decomposition process of polymer mixtures that can be present in plastics waste. The thermal decomposition at low heating rate has been monitored by thermogravimetry/mass spectrometry (TG/MS), while the fast pyrolysis processes have been studied by pyrolysis-gas chromatography/mass spectrometry (Py-GC/MS).

2. Experimental

2.1. Samples

Poly(ethylene terephthalate) (PET) was obtained from Eastman Kodak Co., Ltd and poly(vinyl chloride) (PVC) from Geon Chemical Co. Ltd. Japan. Commercially available acrilonitrile-butadiene-styrene copolymer (ABS) containing brominated epoxy oligomer fl ame retardant and Sb₂O₃ synergist was used in the present investigation. The samples were mechanically mixed to simulate the situation in the municipal waste.

2.2. Methods

2.2.1. Thermogravimetry/mass spectrometry (TG/MS)

The TG/MS system is built of a Perkin-Elmer TGS-2 thermobalance and a HIDEN HAL 2/301 PIC quadrupole mass spectrometer. Typically 0.5-1 mg polymer samples were placed into the platinum sample pan and heated at a 10 °C min^-1 up to 600 °C in argon atmosphere. Portions of the volatile products were introduced into the mass spectrometer through a glass lined metal capillary transfer line heated to 300°. The quadrupole mass spectrometer operated at a 70 eV electron energy.

2.2.2. Pyrolysis–gas chromatography/mass spectrometry (Py–GC/MS)

Pyrolysis experiments were carried out on a Pyroprobe 2000 pyrolyzer (Chemical Data Systems). About 0.5 mg samples were pyrolyzed at 600 °C (calibrated temperature) for 20 s in a quartz tube using helium as a carrier gas. Analysis of the volatile products was accomplished on line with a GC/MS (Agilent Techn. Inc. 6890 GC / 5973 MSD) using HP-5MS capillary column (30 m × 0.25 mm i.d., 0.25 µm fi lm thickness). The pyrolysis interface and the GC injector were kept at 320 and 300 °C, respectively. The GC oven was programmed to hold at 40 °C for 1 min and then increase to 320 °C at a rate of 10 °C

min^-1. The mass spectrometer operated at 70 eV in the EI mode. The mass range of 25-800 Da was scanned.

3. Results and Discussion

Thermogravimetry/mass spectrometry is suitable for the analysis of the decomposition products under slow heating rate. Fig. 1 shows the differential thermogravimetric (DTG) curves of a few polymer mixtures made with ABS. For comparison, Fig. 1a illustrates the DTG curves of the pure polymers and the sum of the DTG curves of the 3 polymers in the ratio of mixing (1:1:1). PVC starts to decompose at the lowest temperature with the evolu-tion of hydrochloric acid and benzene. The second decomposievolu-tion step occurs between 400 and 500°C with the formation of substituted aromatic products. PET decomposes in a single step with the maximum rate of decomposition at 430°C and its decomposition par-tially overlaps with the second decomposition peak of PVC. The decomposition of ABS containing brominated fl ame retardant and Sb₂O₃ synergist occurs in two separated steps.

The fi rst peak can be roughly attributed to the decomposition of the fl ame retardant. The decomposition of ABS overlaps with the decomposition of both PVC and PET.

The DTG curve of the 1:1:1 mixture of the three polymers (Fig. 1b) is strikingly differ-ent from the summed curve of the individual DTG curves of the polymers. The change can not be explained by heat and mass transport problems since we used very small amount of samples (about 1 mg). Furthermore, the higher heat capacity of the mixtures may cause only the shift of the DTG curve, but can not change the shape of the curve completely. As the mass spectrometric curves indicate (not shown) the decomposition of all three polymer components were infl uenced by the presence of other polymers. For a better understanding, the binary mixtures of the same polymers were also measured.

As Fig. 1d and e demonstrate the fi rst peak of PVC becomes sharper either with PET or ABS indicating an enhanced dehydrochlorination rate. As a result, in the ternary mix-ture the dehydrochlorination peak of PVC is shifted to lower temperamix-ture and becomes very sharp. It seems that the second PVC peak (scission of the polymer chains) at around 460°C is not much affected by the presence of other materials. Using low heating rate, PVC does not have signifi cant infl uence on the decomposition rate of PET and the sec-ond peak of ABS. Apparently the dehydrochlorination occurs at lower temperatures and the cleavage of the aliphatic chains of PVC does not interfere with the decomposition of other polymers. However, the fi rst DTG peak of ABS is reduced in the presence of PVC indicating that PVC exerts an effect on the debromination reaction of the fl ame retardant.

As Fig. 1c shows, PET and ABS mutually infl uence the decomposition of each others.

The mass spectrometric analysis revealed that the decomposition of both ABS and PET shifts to lower temperature, but the decomposition rate of the fl ame retardant is not much affected.

Figure 1: Differential thermogravimetric (DTG) curves of (a) the individual polymers and (b, c, d, e) the mixtures in comparison with the calculated curves (sum of the individual polymer curves).

The polymers were mixed in a mass ratio of 1:1 and 1:1:1.

In order to study the formation of high molecular mass products, Py-GC/MS experiments were carried out at 600°C. Fig. 2 shows the pyrogram of the 1:1:1 ternary mixture of PET, PVC and ABS indicating the formation of various chlorinated and brominated products.

The brominated epoxy fl ame retardant releases brominated phenols during pyrolysis. In the pyrolyzate of the mixture, similar amount of mixed chlorinated and brominated phe-nols are found indicating the strong chlorinating capability of the degrading PVC. Chlo-rination and bromination of the PET decomposition products also occurs, mainly the ethylene groups react with the halogen radicals. More chlorinated esters are formed than brominated or mixed halogenated esters, but it can be explained by the fact that the

chlo-Temperature (°C)

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rine content of PVC is higher than the bromine content of ABS. It can be concluded that all the tree polymers infl uence the decomposition of each others and signifi cant amount of various chlorinated and brominated products are released during the decomposition of the mixture.

Figure 2: Total ion chromatogram of the pyrolysis products of the mixture of PET + PVC + ABS.

Acknowledgements

This study was supported by the Hungarian National Research Fund (OTKA No. T037704 and T047377).

References

[1] M.P. Luda, A.I. Balabanovich, G. Camino, J. Anal. Appl. Pyrol., 65 (2002) 25-40.

[2] E. Jakab, Md.A. Uddin, T. Bhaskar, Y. Sakata, J. Anal. Appl. Pyrol., 68-69 (2003) 83-99.

[3] K. Kulesza, K. German, J. Anal. Appl. Pyrol., 67 (2003) 123-134.

[4] M. Blazsó, B. Zelei, and E. Jakab, J. Anal. Appl. Pyrol., 1995, 35, 221.

[5] F. Kubatovics and M. Blazsó, Macromol. Chem. Phys., 2000, 201, 349.

[6] Z. Czégény and M. Blazsó, J. Anal. Appl. Pyrol., 2001, 58-59, 95.

[7] M. Day, J. D. Cooney, C. Touchette-Barrette, and S. E Sheehan, J. Anal. Appl.

Pyrol., 1999, 52, 199.

[8] Z. Czégény, E. Jakab, and M. Blazsó, Macromol. Mater. Eng., 2002, 287, 277.

[9] Y. Sakata, Md.A. Uddin, A. Muto, M. Narazaki, K, Koizumi, K. Murata, M. Kaji, Ind. Eng. Chem. Res., 37 (1998) 2889-2892.

[10] Md.A. Uddin, T. Bhaskar, J. Kaneko, A. Muto, Y. Sakata, T. Matsui, Fuel, 81 (2002) 1819-1825.

[11] T. Bhaskar, J. Kaneko, A. Muto, Y. Sakata, E. Jakab, T. Matsui, Md.A. Uddin, J.

Anal. Appl. Pyrol., 72 (2004) 27-33.

[12] R. Knümann and H. Bockhorn, Combust. Sci. Technol., 1994, 101, 285.