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Mechanochemical dechlorination of polyvinyl chloride by co-grinding with various metal oxides
Advanced Powder Technol., Vol. 15, No. 2, pp. 215– 225 (2004) Ó VSP and Society of Powder Technology, Japan 2004. Also available online - www.vsppub.com
Original paper Mechanochemical dechlorination of polyvinyl chloride by co-grinding with various metal oxides TSUYOSHI INOUE 1;¤ , MIYUKI MIYAZAKI 1 , MASATAKA KAMITANI 1 , JUNYA KANO 2 and FUMIO SAITO 2 1 Sekisui Chemical Co. Ltd., 2-2 Kamichoshi-cho Kamitoba Minami-ku, Kyoto 601-8105, Japan 2 Institute of Multidisciplinary Research for Advanced Materials, Tohoku University,
2-1-1 Katahira Aoba-ku, Sendai 980-8577, Japan Received 6 December 2002; accepted 16 June 2003 Abstract—Polyvinyl chloride powder was ground with one of the oxide powders, i.e. CaO, Fe2 O3 , SiO2 and Al2 O3 , in air by a planetary ball mill to investigate the mechanochemical dechlorination of PVC. Grinding causes the size reduction of the each component. The PVC powder sample is degraded by grinding with reduction of molecular weight, forming C C bonds in its structure. The dechlorination occurs in all the ground mixtures, but its phenomenon is classied into two types. One is the solid-phase reaction to form chlorides, CaOHCl and FeCl2 2H2 O, when CaO and Fe2 O3 powders are used. The other is the release of HCl gas by degradation when SiO2 and Al2 O3 powders are used. All the same, the dechlorination yield increases not only with an increase in grinding time, but also with excess of oxide powder. Keywords: Grinding; polyvinyl chloride; oxide; dechlorination; degradation.
1. INTRODUCTION
Polyvinyl chloride (PVC, [ (CH2 CHCl)n ], n D polymerization degree) has a unique chemical property of high stability for heat and chemicals, and it has a potential to exhibit a wide variety of plastic-elastic properties from exible to rigid PVC products by mixing with plasticizers and additives. In addition, the PVC production industries plays a signicant role in consuming chlorine emitted from soda production industries. Also, PVC products exhibit high cost performance in comparison with other plastics, so a large amount of PVC raw material has been used for industrial, agricultural, medical goods, etc. However, after the usage of such PVC products, they would be wastes, which are disposed of mainly by reclamation and combustion. The former needs a huge land area, which is not ¤
To whom correspondence should be addressed. E-mail: [email protected]
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always available. The latter has been carried out in waste disposal facilities for a long period of time, but is now applied under restricted conditions because of the emission of harmful substances such as HCl gas and kinds of PCDD and PCDF (the dioxin group). The establishment of appropriate treatment of PVC wastes is required. Recently, several methods for disposing of PVC wastes have been proposed — the rst is the blast furnace method [1], the second is the rotary kiln method [2] and the third is the liquefying method [3]. These are unique and excellent, however, based on heating operation, they have difculties in treating generated HCl gas without any problem, and so make the instruments complicated and the treating cost high. Zhang et al. have proposed a novel method to treat waste PVC by a grinding technique with CaO, followed by washing with water [4]. This is mainly a grinding operation at room temperature and chlorine is removed from PVC through the formation of CaOHCl. Saeki et al. have also investigated a similar process with CaO, CaCO3 , and blast furnace slag that is mainly composed of CaO, Fe2 O3 , SiO2 and Al2 O3 [5]. They have insisted on the effectiveness of CaO and the blast furnace slag, but the mechanism of dechlorination is still unclear when the blast furnace slag is used. The main purpose of this paper is to provide information on the mechanochemical dechlorination of PVC in co-grinding with one of CaO, Fe2 O3 , SiO 2 and Al2 O3 .
2. EXPERIMENT
2.1. Samples The PVC powder sample used in this experiment was a chemical reagent (Wako Chemical) and its initial mean particle diameter was about 133 ¹m. The polymerization degree of the PVC sample was about 1100. CaO, Fe2 O 3 , SiO 2 and Al2 O3 were also chemical reagents (Wako Chemical), and the mean particle sizes were 13.5 ¹m for CaO, 1.3 ¹m for Fe2 O3 , 600 ¹m for SiO2 and 34 ¹m for Al2 O 3 . The PVC powder sample was mixed with one of the oxide powders (CaO, Fe2 O3 , SiO2 and Al2 O 3 ) at different molar ratios of metal in the oxide to Cl in PVC. 2.2. Grinding A planetary ball mill (Pulverisette-7; Fritsch) was used to cause the mechanochemical reaction of the PVC sample with the oxide. The mill consists of a pair of pots made of stainless steel and they are rotated on the revolution disk at the same speed (600 r.p.m.). The inner diameter and depth of the pot were both 40 mm, and the volume was approximately 50 cm3 . Seven steel balls (diameter 15 mm) were put into each pot together with 3.0 g of the mixed powder, the mill was started to run under the atmospheric conditions for different periods of time and the product was thoroughly removed from each pot after the grinding.
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2.3. Characterization The mean size of the ground product was determined by measuring its size distribution curve using a particle size distribution analyzer (Seishin; PRO-7000S). X-ray diffraction (XRD) analysis (Rigaku; RINT-1100 Cu-Ka) was carried out for the ground mixtures to determine the phases and chemical compositions. Infrared absorption analysis (Perkin-Elmer; Spectrum One) and gel permeation chlomatography (GPC) analysis (Hitachi; L-3300RI) were carried out to investigate the chemical structure and molecular weight of the PVC ingredient in the ground products. Films for infrared absorption analysis were prepared as follows. The ground product (1.5 g) was dispersed in distilled water (250 ml), being stirred for 1 h to extract soluble inorganic compounds included in it. The slurry was ltered off and the residue was dried at 60 ± C for 24 h. The dried residue was dispersed in tetrahydrofurane (THF) solution to dissolve soluble the PVC ingredient. The solution was pored on a plate made of NaCl to remove THF thoroughly by heating, forming the lm. GPC analysis was applied to the solution at room temperature and the molecular weight of the PVC ingredient was calculated on the basis of polystyrene as a standard. Figure 1 shows the ow chart of the experiment and the characterization. 2.4. Dechlorination measurement Dechlorination of the PVC sample was determined by an ion chromatograph (IC) (LC10 series; Shimadzu) as follows. A sample of 0.5 g ground product was dispersed in 50 ml distilled water and stirred for 1 h to extract the soluble compounds. The slurry was then ltered off and the ltrate was subjected to IC analysis to determine the chlorine concentration in the ltrate.
3. RESULTS AND DISCUSSION
3.1. Change in mean size of the ground product Figure 2 shows the mean size of the ground product as a function of grinding time for all the mixing systems. The mixtures were prepared at an equi-molar ratio of Cl in PVC to metal in one of the oxides. The initial size of the each sample is indicated. The minimum mean size of the ground product is measured in the PVC–SiO2 system, although the initial size of SiO 2 is the largest (600 ¹m) of all oxides. Moreover, the size of the ground product for the PVC–Al2 O3 system is small, although the initial size of Al2 O3 is the second largest one (34 ¹m). These ndings imply that the size of the ground product is not always dependent on the initial size of the oxide sample in the mixture, when SiO2 and Al2 O3 play a role as a grinding aid. On the other hand, the size change of the ground product in the PVC– CaO/Fe2 O3 systems is a little different from that in the PVC–SiO2 /Al2 O3 systems. This may be due to the reaction system between PVC and CaO/Fe2 O3 , which differs
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Figure 1. Flow chart of the experiment and the characterization.
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Figure 2. Mean size of the ground product as a function of grinding time for all the mixing systems (Cl in PVC : metal in oxide D 1 : 1).
from the PVC–SiO 2 /Al2 O3 systems. This will be discussed in later. In any case, the mean size of the ground product tends to decrease with the grinding time. 3.2. Dechlorination of PVC in the ground mixtures Figure 3 shows XRD patterns of the four kinds of mixture ground for 6 h. When the PVC sample is ground with either CaO or Fe2 O3 , it is found that CaOHCl or FeCl2 2H2 O is formed in the ground product. This implies that the grinding causes a mechanochemical reaction between the two samples. In addition, for the system PVC and Fe2 O3 , FeO is formed in the product, suggesting that PVC has reduced Fe2 O3 into FeO through the mechanochemical reaction. The reaction mechanism will be discussed later. On the contrary, when the PVC sample is ground with either SiO2 or Al2 O3 , no peaks of chloride are observed in the XRD pattern, while a peak of Fe is observed. This Fe peak would be due to the wear from the mill pot and balls during the grinding. It is noted that the inner pressure of the pot after the operation was increased and HCl gas was detected by a gas detector (Komei Rikagaku). This may be due to the degradation of PVC by grinding with SiO2 and Al2 O3 , which play signicant roles as grinding aids, not a reactant, because these oxides are chemically stable against PVC under these grinding conditions.
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Figure 3. XRD patterns of the four kinds of mixture [(a) PVC–CaO, (b) PVC–Fe2 O3 , (c) PVC–SiO2 and (d) PVC–Al2 O3 ] ground for 6 h (Cl in PVC : metal in oxide D 1 : 1).
Figure 4 shows IR spectra of the PVC (a) as a reference and the PVC ingredient in the four kinds of product ground for 6 h. In the spectra (b–e), in addition to PVC derived peaks (spectrum a), a new weak peak can be observed at 1680 cm¡1 corresponding to the C C stretch mode. This clearly indicates that C C bonds exist in the structure of the PVC ingredient in all the ground products. The C C bonds may be formed by the removal of HCl from PVC during the degradation. The initiation of the removal of HCl from PVC would be caused by the mechanochemical interaction between Cl in PVC and metal in the oxide. According to the computer chemistry work done by Mizukami et al. [6], the bonding energy of C Cl in PVC is the weakest one of the all, so that the Cl in PVC would be easily removed by the external force by interacting with other elements such as metal in the oxide. Taking the experimental results shown in Fig. 3 into account, as Zhang et al. has already indicated [4], the reaction of PVC with CaO can be described as follows, based on the monomer unit: .CH 2 CHCl/
C CaO !
.CH CH/
C CaOHCl
(1)
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Figure 4. IR spectra of the PVC (a) and the PVC ingredient in the four kinds of mixture [(b) PVC– CaO, (c) PVC–Fe2 O3 , (d) PVC–SiO2 and (e) PVC–Al2 O3 ] ground for 6 h (Cl in PVC : metal in oxide D 1 : 1).
Regarding the reaction of PVC with Fe2 O3 , it can be described by (2), judging from the ground products: 4 .CH2 CHCl/
C Fe2 O 3 ! 3 .CH CH/ C .CHClCHCl/ C FeCl2 2H2 O C FeO
(2)
Equation (2) can be made up of (3) and (4). In (4), it is well known that the additional reaction of X2 (X Cl, Br) to an alkene takes place easier than that of HX to an alkene, and which proceeds easily at room temperature [7]. C Fe2 O3 ! 4 .CH CH/ C FeCl2 2H2 O C FeO C Cl2 .CH CH/ C Cl2 ! .CHClCHCl/
4 .CH2 CHCl/
(3) (4)
As for the systems of PVC–SiO2 /Al2 O3 , the interaction during the grinding may take place as follows: .CH2 CHCl/ C SiO2 ! .CH2 CHCl/ C Al2 O3 !
.CH CH/ .CH CH/
C SiO2 C HCl C Al2 O 3 C HCl
(5) (6)
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Figure 5. Molecular weight of the PVC ingredient in the mixtures as a function of grinding time (Cl in PVC : metal in oxide D 1 : 1).
Figure 6. Dechlorination of PVC in the mixtures as a function of grinding time for the PVC– CaO/Fe2 O3 systems (Cl in PVC : metal in oxide D 1 : 1).
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Figure 5 shows the molecular weight of the PVC ingredient in the mixtures as a function of grinding time. The molecular weight decreases with grinding time, although the gradient of the slope is not the same. The most signicant change is seen in the PVC–Fe2 O3 system. This would be attributed to the unique mechanism of the PVC–Fe2 O3 system as shown in (2)–(4), i.e. Fe reacts with two Cls in PVC and this time the main structure of PVC would be cut. The second and the third places are the PVC–SiO2 /Al2 O3 systems, and this implies that SiO2 /Al2 O3 act as grinding aids, leading to degradation of the structure of PVC. The most moderate change in molecular decrease is the PVC–CaO system, so that CaO would react predominantly with Cl in the PVC to form chloride (CaOHCl) with degradation of the structure of PVC. Figure 6 shows the dechlorination of PVC in the mixtures as a function of grinding time for the PVC–CaO/Fe2 O3 systems. As for the mixtures of the PVC–SiO2 /Al2 O3 systems, the data is not shown in Fig. 6, due to the different dechlorination phenomena from those in the PVC–CaO/Fe2 O3 systems. The dechlorination increases with the grinding time in both mixing systems. The dechlorination value of the PVC–Fe2 O3 system is about twice as large as that of the PVC–CaO system. This is because the amount of Cl reacting with Fe is two times larger than that with Ca, as has been deduced from (1) and (2), and this was conrmed by the preliminary experiment. In addition, this phenomenon corresponds to the result that the mean particle size of the ground product in the PVC–Fe2 O3 system is smaller than that in the PVC–CaO system. The dechlorination of PVC in the PVC–SiO2 /Al2 O 3 systems has also occurred, but it is a different process that is the release of HCl gas with degradation of PVC. Figure 7 shows the dechlorination of PVC in the mixtures as a function of the molar ratio of the metal in oxide to Cl in PVC for the PVC–CaO/Fe2 O3 systems ground for 6 h. The dechlorination increases with the molar ratio in both systems. This may be because the contact frequency of PVC with the oxide increases. The amount of additive to the PVC would play a signicant role in the control of the mechanochemical dechlorination of PVC.
4. CONCLUSION
PVC powder was ground with oxide powder to clarify the difference in its mechanochemical dechlorination. Four kinds of oxide powders were chosen, i.e. CaO, Fe2 O3 , SiO2 and Al2 O3 . The mixtures were ground by a planetary ball mill, and then the products were analyzed by a particle size distribution analyzer, XRD, IR, GPC and ion chromatography. The experimental results are summarized as follows. (i) Grinding causes a reduction of particle size of the mixture in all of the mixing systems. (ii) Chlorides (CaOHCl, FeCl2 2H2 O) are formed in the products produced by grinding the mixtures of PVC–CaO/Fe2 O3 , respectively, whereas no chlorides
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Figure 7. Dechlorination of PVC in the mixtures as a function of the molar ratio of the metal in oxide to Cl in PVC for PVC–CaO/Fe2O 3 systems ground for 6 h.
except HCl gas are formed when SiO2 and Al2 O 3 are used as the oxide. SiO2 and Al2 O3 powders play a signicant role as grinding aids. (iii) C C bonds are formed in the structure of the PVC ingredient in the ground product in all the mixing systems. The PVC is degraded by the grinding, decreasing in its molecular weight. (iv) The reduction of molecular weight of PVC ingredient in the co-grinding with Fe2 O3 is the most signicant, and the second and the third places belong to the PVC–SiO2 /Al2 O3 mixing systems. The most moderate change in molecular decrement is for the PVC–CaO mixing system, implying that CaO reacts predominantly with Cl in the PVC. (v) The dechlorination mechanism in the present systems is classied into two groups — one is the reaction system like PVC–CaO/Fe2 O3 , forming metal chloride; the other is the non-reaction system like PVC–SiO2 /Al2 O3 that releases HCl gas. All the same, dechlorination of PVC increases with an increase in both grinding time and additive ratio.
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REFERENCES 1. T. Fukumoto, in: Disposal Technology of Wastes Materials, pp. 53–77. Kyoritsu Shuppan, Tokyo (1977). 2. K. Furubayashi, Disposing technologies by means of RDF (refuse derived fuel), Kagaku-Kogaku 61, 502–505 (1997). 3. K. Murata, Technologies for liquefying waste plastics, Kagaku Kogaku 61, 510–512 (1997). 4. Q. Zhang, F. Saito, K. Shimme and S. Masuda, Dechlorination of PVC by a mechanochemical treatment under atmospheric condition, J. Soc. Powder. Technol., Japan 36, 468–473 (1999). 5. S. Saeki, J. Kano, K. Shimme, S. Masuda and T. Inoue, Effect of additives on dechlorination of PVC by mechanochemical treatment, J. Mater. Cycles Waste Manag. 3, 20–23 (2001). 6. K. Mizukami, T. Ito, Q. Zhang, F. Saito and H. J. Ryu, Computer simulation study on the interaction between a PVC model molecule and Ca(OH)2 excited under mechanical force, Environ. Sci. Engng (submitted). 7. R. T. Morrison and R. N. Boyd, in: Organic Chemistry, 6th edn, pp. 439–443. Tokyokagakudoujin, Tokyo (1994) (in Japanese).
Original paper Mechanochemical dechlorination of polyvinyl chloride by co-grinding with various metal oxides TSUYOSHI INOUE 1;¤ , MIYUKI MIYAZAKI 1 , MASATAKA KAMITANI 1 , JUNYA KANO 2 and FUMIO SAITO 2 1 Sekisui Chemical Co. Ltd., 2-2 Kamichoshi-cho Kamitoba Minami-ku, Kyoto 601-8105, Japan 2 Institute of Multidisciplinary Research for Advanced Materials, Tohoku University,
2-1-1 Katahira Aoba-ku, Sendai 980-8577, Japan Received 6 December 2002; accepted 16 June 2003 Abstract—Polyvinyl chloride powder was ground with one of the oxide powders, i.e. CaO, Fe2 O3 , SiO2 and Al2 O3 , in air by a planetary ball mill to investigate the mechanochemical dechlorination of PVC. Grinding causes the size reduction of the each component. The PVC powder sample is degraded by grinding with reduction of molecular weight, forming C C bonds in its structure. The dechlorination occurs in all the ground mixtures, but its phenomenon is classied into two types. One is the solid-phase reaction to form chlorides, CaOHCl and FeCl2 2H2 O, when CaO and Fe2 O3 powders are used. The other is the release of HCl gas by degradation when SiO2 and Al2 O3 powders are used. All the same, the dechlorination yield increases not only with an increase in grinding time, but also with excess of oxide powder. Keywords: Grinding; polyvinyl chloride; oxide; dechlorination; degradation.
1. INTRODUCTION
Polyvinyl chloride (PVC, [ (CH2 CHCl)n ], n D polymerization degree) has a unique chemical property of high stability for heat and chemicals, and it has a potential to exhibit a wide variety of plastic-elastic properties from exible to rigid PVC products by mixing with plasticizers and additives. In addition, the PVC production industries plays a signicant role in consuming chlorine emitted from soda production industries. Also, PVC products exhibit high cost performance in comparison with other plastics, so a large amount of PVC raw material has been used for industrial, agricultural, medical goods, etc. However, after the usage of such PVC products, they would be wastes, which are disposed of mainly by reclamation and combustion. The former needs a huge land area, which is not ¤
To whom correspondence should be addressed. E-mail: [email protected]
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T. Inoue et al.
always available. The latter has been carried out in waste disposal facilities for a long period of time, but is now applied under restricted conditions because of the emission of harmful substances such as HCl gas and kinds of PCDD and PCDF (the dioxin group). The establishment of appropriate treatment of PVC wastes is required. Recently, several methods for disposing of PVC wastes have been proposed — the rst is the blast furnace method [1], the second is the rotary kiln method [2] and the third is the liquefying method [3]. These are unique and excellent, however, based on heating operation, they have difculties in treating generated HCl gas without any problem, and so make the instruments complicated and the treating cost high. Zhang et al. have proposed a novel method to treat waste PVC by a grinding technique with CaO, followed by washing with water [4]. This is mainly a grinding operation at room temperature and chlorine is removed from PVC through the formation of CaOHCl. Saeki et al. have also investigated a similar process with CaO, CaCO3 , and blast furnace slag that is mainly composed of CaO, Fe2 O3 , SiO2 and Al2 O3 [5]. They have insisted on the effectiveness of CaO and the blast furnace slag, but the mechanism of dechlorination is still unclear when the blast furnace slag is used. The main purpose of this paper is to provide information on the mechanochemical dechlorination of PVC in co-grinding with one of CaO, Fe2 O3 , SiO 2 and Al2 O3 .
2. EXPERIMENT
2.1. Samples The PVC powder sample used in this experiment was a chemical reagent (Wako Chemical) and its initial mean particle diameter was about 133 ¹m. The polymerization degree of the PVC sample was about 1100. CaO, Fe2 O 3 , SiO 2 and Al2 O3 were also chemical reagents (Wako Chemical), and the mean particle sizes were 13.5 ¹m for CaO, 1.3 ¹m for Fe2 O3 , 600 ¹m for SiO2 and 34 ¹m for Al2 O 3 . The PVC powder sample was mixed with one of the oxide powders (CaO, Fe2 O3 , SiO2 and Al2 O 3 ) at different molar ratios of metal in the oxide to Cl in PVC. 2.2. Grinding A planetary ball mill (Pulverisette-7; Fritsch) was used to cause the mechanochemical reaction of the PVC sample with the oxide. The mill consists of a pair of pots made of stainless steel and they are rotated on the revolution disk at the same speed (600 r.p.m.). The inner diameter and depth of the pot were both 40 mm, and the volume was approximately 50 cm3 . Seven steel balls (diameter 15 mm) were put into each pot together with 3.0 g of the mixed powder, the mill was started to run under the atmospheric conditions for different periods of time and the product was thoroughly removed from each pot after the grinding.
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2.3. Characterization The mean size of the ground product was determined by measuring its size distribution curve using a particle size distribution analyzer (Seishin; PRO-7000S). X-ray diffraction (XRD) analysis (Rigaku; RINT-1100 Cu-Ka) was carried out for the ground mixtures to determine the phases and chemical compositions. Infrared absorption analysis (Perkin-Elmer; Spectrum One) and gel permeation chlomatography (GPC) analysis (Hitachi; L-3300RI) were carried out to investigate the chemical structure and molecular weight of the PVC ingredient in the ground products. Films for infrared absorption analysis were prepared as follows. The ground product (1.5 g) was dispersed in distilled water (250 ml), being stirred for 1 h to extract soluble inorganic compounds included in it. The slurry was ltered off and the residue was dried at 60 ± C for 24 h. The dried residue was dispersed in tetrahydrofurane (THF) solution to dissolve soluble the PVC ingredient. The solution was pored on a plate made of NaCl to remove THF thoroughly by heating, forming the lm. GPC analysis was applied to the solution at room temperature and the molecular weight of the PVC ingredient was calculated on the basis of polystyrene as a standard. Figure 1 shows the ow chart of the experiment and the characterization. 2.4. Dechlorination measurement Dechlorination of the PVC sample was determined by an ion chromatograph (IC) (LC10 series; Shimadzu) as follows. A sample of 0.5 g ground product was dispersed in 50 ml distilled water and stirred for 1 h to extract the soluble compounds. The slurry was then ltered off and the ltrate was subjected to IC analysis to determine the chlorine concentration in the ltrate.
3. RESULTS AND DISCUSSION
3.1. Change in mean size of the ground product Figure 2 shows the mean size of the ground product as a function of grinding time for all the mixing systems. The mixtures were prepared at an equi-molar ratio of Cl in PVC to metal in one of the oxides. The initial size of the each sample is indicated. The minimum mean size of the ground product is measured in the PVC–SiO2 system, although the initial size of SiO 2 is the largest (600 ¹m) of all oxides. Moreover, the size of the ground product for the PVC–Al2 O3 system is small, although the initial size of Al2 O3 is the second largest one (34 ¹m). These ndings imply that the size of the ground product is not always dependent on the initial size of the oxide sample in the mixture, when SiO2 and Al2 O3 play a role as a grinding aid. On the other hand, the size change of the ground product in the PVC– CaO/Fe2 O3 systems is a little different from that in the PVC–SiO2 /Al2 O3 systems. This may be due to the reaction system between PVC and CaO/Fe2 O3 , which differs
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Figure 1. Flow chart of the experiment and the characterization.
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Figure 2. Mean size of the ground product as a function of grinding time for all the mixing systems (Cl in PVC : metal in oxide D 1 : 1).
from the PVC–SiO 2 /Al2 O3 systems. This will be discussed in later. In any case, the mean size of the ground product tends to decrease with the grinding time. 3.2. Dechlorination of PVC in the ground mixtures Figure 3 shows XRD patterns of the four kinds of mixture ground for 6 h. When the PVC sample is ground with either CaO or Fe2 O3 , it is found that CaOHCl or FeCl2 2H2 O is formed in the ground product. This implies that the grinding causes a mechanochemical reaction between the two samples. In addition, for the system PVC and Fe2 O3 , FeO is formed in the product, suggesting that PVC has reduced Fe2 O3 into FeO through the mechanochemical reaction. The reaction mechanism will be discussed later. On the contrary, when the PVC sample is ground with either SiO2 or Al2 O3 , no peaks of chloride are observed in the XRD pattern, while a peak of Fe is observed. This Fe peak would be due to the wear from the mill pot and balls during the grinding. It is noted that the inner pressure of the pot after the operation was increased and HCl gas was detected by a gas detector (Komei Rikagaku). This may be due to the degradation of PVC by grinding with SiO2 and Al2 O3 , which play signicant roles as grinding aids, not a reactant, because these oxides are chemically stable against PVC under these grinding conditions.
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Figure 3. XRD patterns of the four kinds of mixture [(a) PVC–CaO, (b) PVC–Fe2 O3 , (c) PVC–SiO2 and (d) PVC–Al2 O3 ] ground for 6 h (Cl in PVC : metal in oxide D 1 : 1).
Figure 4 shows IR spectra of the PVC (a) as a reference and the PVC ingredient in the four kinds of product ground for 6 h. In the spectra (b–e), in addition to PVC derived peaks (spectrum a), a new weak peak can be observed at 1680 cm¡1 corresponding to the C C stretch mode. This clearly indicates that C C bonds exist in the structure of the PVC ingredient in all the ground products. The C C bonds may be formed by the removal of HCl from PVC during the degradation. The initiation of the removal of HCl from PVC would be caused by the mechanochemical interaction between Cl in PVC and metal in the oxide. According to the computer chemistry work done by Mizukami et al. [6], the bonding energy of C Cl in PVC is the weakest one of the all, so that the Cl in PVC would be easily removed by the external force by interacting with other elements such as metal in the oxide. Taking the experimental results shown in Fig. 3 into account, as Zhang et al. has already indicated [4], the reaction of PVC with CaO can be described as follows, based on the monomer unit: .CH 2 CHCl/
C CaO !
.CH CH/
C CaOHCl
(1)
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Figure 4. IR spectra of the PVC (a) and the PVC ingredient in the four kinds of mixture [(b) PVC– CaO, (c) PVC–Fe2 O3 , (d) PVC–SiO2 and (e) PVC–Al2 O3 ] ground for 6 h (Cl in PVC : metal in oxide D 1 : 1).
Regarding the reaction of PVC with Fe2 O3 , it can be described by (2), judging from the ground products: 4 .CH2 CHCl/
C Fe2 O 3 ! 3 .CH CH/ C .CHClCHCl/ C FeCl2 2H2 O C FeO
(2)
Equation (2) can be made up of (3) and (4). In (4), it is well known that the additional reaction of X2 (X Cl, Br) to an alkene takes place easier than that of HX to an alkene, and which proceeds easily at room temperature [7]. C Fe2 O3 ! 4 .CH CH/ C FeCl2 2H2 O C FeO C Cl2 .CH CH/ C Cl2 ! .CHClCHCl/
4 .CH2 CHCl/
(3) (4)
As for the systems of PVC–SiO2 /Al2 O3 , the interaction during the grinding may take place as follows: .CH2 CHCl/ C SiO2 ! .CH2 CHCl/ C Al2 O3 !
.CH CH/ .CH CH/
C SiO2 C HCl C Al2 O 3 C HCl
(5) (6)
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Figure 5. Molecular weight of the PVC ingredient in the mixtures as a function of grinding time (Cl in PVC : metal in oxide D 1 : 1).
Figure 6. Dechlorination of PVC in the mixtures as a function of grinding time for the PVC– CaO/Fe2 O3 systems (Cl in PVC : metal in oxide D 1 : 1).
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Figure 5 shows the molecular weight of the PVC ingredient in the mixtures as a function of grinding time. The molecular weight decreases with grinding time, although the gradient of the slope is not the same. The most signicant change is seen in the PVC–Fe2 O3 system. This would be attributed to the unique mechanism of the PVC–Fe2 O3 system as shown in (2)–(4), i.e. Fe reacts with two Cls in PVC and this time the main structure of PVC would be cut. The second and the third places are the PVC–SiO2 /Al2 O3 systems, and this implies that SiO2 /Al2 O3 act as grinding aids, leading to degradation of the structure of PVC. The most moderate change in molecular decrease is the PVC–CaO system, so that CaO would react predominantly with Cl in the PVC to form chloride (CaOHCl) with degradation of the structure of PVC. Figure 6 shows the dechlorination of PVC in the mixtures as a function of grinding time for the PVC–CaO/Fe2 O3 systems. As for the mixtures of the PVC–SiO2 /Al2 O3 systems, the data is not shown in Fig. 6, due to the different dechlorination phenomena from those in the PVC–CaO/Fe2 O3 systems. The dechlorination increases with the grinding time in both mixing systems. The dechlorination value of the PVC–Fe2 O3 system is about twice as large as that of the PVC–CaO system. This is because the amount of Cl reacting with Fe is two times larger than that with Ca, as has been deduced from (1) and (2), and this was conrmed by the preliminary experiment. In addition, this phenomenon corresponds to the result that the mean particle size of the ground product in the PVC–Fe2 O3 system is smaller than that in the PVC–CaO system. The dechlorination of PVC in the PVC–SiO2 /Al2 O 3 systems has also occurred, but it is a different process that is the release of HCl gas with degradation of PVC. Figure 7 shows the dechlorination of PVC in the mixtures as a function of the molar ratio of the metal in oxide to Cl in PVC for the PVC–CaO/Fe2 O3 systems ground for 6 h. The dechlorination increases with the molar ratio in both systems. This may be because the contact frequency of PVC with the oxide increases. The amount of additive to the PVC would play a signicant role in the control of the mechanochemical dechlorination of PVC.
4. CONCLUSION
PVC powder was ground with oxide powder to clarify the difference in its mechanochemical dechlorination. Four kinds of oxide powders were chosen, i.e. CaO, Fe2 O3 , SiO2 and Al2 O3 . The mixtures were ground by a planetary ball mill, and then the products were analyzed by a particle size distribution analyzer, XRD, IR, GPC and ion chromatography. The experimental results are summarized as follows. (i) Grinding causes a reduction of particle size of the mixture in all of the mixing systems. (ii) Chlorides (CaOHCl, FeCl2 2H2 O) are formed in the products produced by grinding the mixtures of PVC–CaO/Fe2 O3 , respectively, whereas no chlorides
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Figure 7. Dechlorination of PVC in the mixtures as a function of the molar ratio of the metal in oxide to Cl in PVC for PVC–CaO/Fe2O 3 systems ground for 6 h.
except HCl gas are formed when SiO2 and Al2 O 3 are used as the oxide. SiO2 and Al2 O3 powders play a signicant role as grinding aids. (iii) C C bonds are formed in the structure of the PVC ingredient in the ground product in all the mixing systems. The PVC is degraded by the grinding, decreasing in its molecular weight. (iv) The reduction of molecular weight of PVC ingredient in the co-grinding with Fe2 O3 is the most signicant, and the second and the third places belong to the PVC–SiO2 /Al2 O3 mixing systems. The most moderate change in molecular decrement is for the PVC–CaO mixing system, implying that CaO reacts predominantly with Cl in the PVC. (v) The dechlorination mechanism in the present systems is classied into two groups — one is the reaction system like PVC–CaO/Fe2 O3 , forming metal chloride; the other is the non-reaction system like PVC–SiO2 /Al2 O3 that releases HCl gas. All the same, dechlorination of PVC increases with an increase in both grinding time and additive ratio.
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