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Energy & Fuels 2003, 17, 896-900
Dehydrochlorination of PVC Materials at High Temperature Xue-Gang Zheng,* Li-Hua Tang, Na Zhang, Qing-Hua Gao, Cheng-Fang Zhang, and Zi-Bin Zhu Research Institute of Chemical Technology, P.O. Box 274, East China University of Science and Technology, 130 Meilong Road, Shanghai 200237, China Received June 5, 2002
A relatively large laboratory-scale sample of a commercial type of PVC material usually used in building, flooring, home furnishings, clothing, etc., is thermally decomposed under carefully controlled conditions. Dehydrochlorination of PVC material at high temperature (up to 600 °C) has been studied. The results show that dehydrochlorination beginning at the temperature of 150 °C can be divided into three stages: low-temperature moderation in the temperature interval 150-180 °C, medium-temperature acceleration in the interval 180-250 °C, and high-temperature moderation in the interval 250-280 °C. The release of chlorine as HCl from PVC materials, at the same temperature, is highest in oxygen, and it increases with an increase of oxygen content. The release of chlorine as HCl is higher by about 5-10% in hydrogen than in nitrogen. In addition, the effect of additives such as dioctyl phthalate (DOP) and calcium stearate (CaSt2) on dehydrochlorination of PVC is studied. The release of chlorine as HCl is markedly reduced by additives.
Introduction Refuse incineration has been widely used all over the world. Dioxin, being very toxic, and carcinogenic compounds related to the content of chloride in refuse are mainly from the incineration of refuse. A survey1 of incinerator operation shows that reducing the content of chloride in refuse can control the formation of dioxin. A simulation test2-4 also shows that dioxin increases with the increasing content of poly(vinyl chloride) or chloride-containing materials. Thus, it is important to study the dehydrochlorination of PVC. Several detailed surveys on the thermal degradation of PVC have been published. McNeill et al.5 conducted the pyrolysis of PVC under continuous evacuation (initial vacuum 10-5 mmHg) at a heating rate of 10 °C/min up to 500 °C, when a hold time of 20 min was allowed. PVC shows two stages of degradation: during the first stage, between 200 and 360 °C, mainly HCl and benzene are released. Very little HCl and benzene are formed in the second stage of degradation, between 360 and 500 °C. An amount of 10% of the Cl remained trapped in the polymer until higher degradation temperatures, giving rise then to the chlorinated compounds * Corresponding author. (1) Yulin, W.; Yishu, Q. Proceedings of the Sixth Mainland-Taiwan Environmental Protection Conference, Gaoxiong, Taiwan, 1999. (2) Kawakami, I., et al. Reduction of PCDDs and PCDFs emissions from an MSW incineration plant. J. Organohalogen Compd. 1997, 31, 393-396. (3) Costner, P. T. Correlation of chlorine input and dioxin output from combustors. A review and reanalysis. J. Organohalogen Compds. 1998, 32, 436-440. (4) Kawakami, Isamu, et al. Reduction of dioxin emissions by the continuous operation of intermittent incinerators. J. Organohalogen Compds. 1997, 31, 401-404. (5) Mcneill, I. C.; Cole, W. J.; Memetea, L. A study of the products of PVC thermal degradation. J. Polym. Degrad. Stabil. 1995, 49, 181191.
which account for 0.14% of the polymer. Chang and Salovey6 examined the thermal degradation of PVC in a helium atmosphere using quartz tubes; the heating rate was 1000 °C/s. At 160 °C, the amount of HCl released was very similar for all the samples and constituted less than 1% of the stoichiometric value in the polymer. At 210 °C, the HCl evolution for all the samples increased, which may be attributed to the activating effect of allylic groups formed during the 160 °C pyrolysis. At 275 °C, more HCl was removed from the polymer chains. When the polymer temperature was raised to 340 °C, the PVC dehydrochlorinated rapidly to form polyene chains which simultaneously yielded benzene through cyclization. At 425 and 500 °C, the residual HCl was removed and longer polymer units broke off, forming higher aromatic products. McNeill & Memetea7 conducted the pyrolysis of PVC, dioctyl phthalate (DOP), and their mixture in air and an inert atmosphere. The results indicated that DOP itself could have a stabilizing effect on the PVC dehydrochlorination. Lattimer and Kroenke8 carried out the pyrolysis of PVC at 550 °C for 20 s by using a platinum coil probe. Their findings showed that direct scission of the PVC chain to form chlorine-containing pyrolysates is an unimportant degradation pathway. Most chlorinecontaining pyrolysates from PVC are due to secondary reactions of HCl with the environment or with non-PVC compounding ingredients. In addition, they found that (6) Chang, E. P.; Salovey, R. Pyrolysis of poly(vinyl chloride). J. Polym. Sci., Polym. Chem. Ed. 1974, 12, 2927-2941. (7) Mcneill, I. C.; Memetea, L. Pyrolysis product of poly(vinyl chloride), dioctyl phthalate and their mixture. J. Polym. Degrad. Stabil. 1994, 43, 9-25. (8) Lattimer, R. P.; Kroenke, W. J. The formation of volatile pyrolysates from poly(vinyl chloride). J. Appl. Polym. Sci. 1980, 25, 101-110.
10.1021/ef020131g CCC: $25.00 © 2003 American Chemical Society Published on Web 05/29/2003
Dehydrochlorination of PVC Materials
Energy & Fuels, Vol. 17, No. 4, 2003 897
Table 1. Ultimate Analysis of Six Commercial PVC Products (wt %) PVC
PPVC
TPVC
FPVC
SPVC
LPVC
WPVC
C H Cl
38.4 4.8 56.8
36.9 4.6 46.5
45.8 5.72 39.05
35.88 4.15 34.51
50.1 6.3 24.79
29.19 3.3 28.66
the presence of oxygen could promote the abstraction of hydrogen from PVC chains or else the rechlorination of polyene chains. Alajbeg9 performed nonflaming combustion of PVC at high temperature. The results showed that the residues were expected to incorporate some chlorine, because the organic volatile-condensable fraction contained a low portion of chlorine-containing compounds and because the total mass of the gaseous fraction (including HCl) accounted for 27% to 48% of the initial sample mass (the ratio of the chlorine weight to the weight of the rest of the vinyl chloride molecule is roughly 56.5:43.5). As can be seen, most literature related to the topic of the present work is about pyrolysis of PVC, such as the procedure of pyrolysis, the effect of atmosphere on pyrolysis, analysis of pyrolysates, and the behavior of dehydrochlorination of PVC at high temperature, has not received much attention. This type of research is rather scarce in the literature. Particularly, the research on thermal degradation of PVC in hydrogen and water vapor cannot be found in the literature, even as it has been recently reviewed.10-12 For these reasons, in this paper a relatively large laboratory-scale sample of six PVC materials is submitted to decomposition in a flowtube reactor at a heating rate of 1000 °C/s at high temperature. The relations between temperature and dehydrochlorination are studied under a variety of conditions: atmospheres of nitrogen, hydrogen, oxygen, air, and aqueous vapor, or in the presence of additives including dioctyl phthalate (DOP) and calcium stearate (CaSt2). Experimental Section Materials. Six commercial PVC products are selected for this study. They are pure PVC resin (PPVC)(in powdered form,
