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Ind. Eng. Chem. Res. 2003, 42, 5018-5023
Mechanochemical Decomposition of an Aromatic Polyamide Film Yasumitsu Tanaka,† Qiwu Zhang,*,‡ and Fumio Saito‡ RMC, CNC, Sony Company Ltd., Sendai Technology Center, 3-4-1 Sakuragi, Tagajo 985-0842, Japan, and IMRAM, Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai 980-8577, Japan
A film sample of aromatic polyamide, or so-called Aramid, was subjected to grinding in air with calcium oxide (CaO) powder by using a planetary ball mill, to investigate its mechanochemical decomposition. The reaction was found to proceed with an increase in the period of grinding time, and all chlorine in the film was transformed into inorganic chlorides in prolonged grinding. The process was characterized by X-ray diffraction analysis, thermogravimetric analysis, scanning electron microscopic observation, and Raman shift analysis. The main products after decomposition reaction were water-soluble calcium chloride hydrate, amorphous carbon, calcium carbonate, and a small part of calcium nitrite, as a result of a rupture in the covalent bonds of C-C, C-Cl, C-N, and C-H in aramid film construction into some new bondings. The most significant phenomenon was the formation of Ca(NO2)2, suggesting the strong oxidative ability of CaO during the mechanochemical reaction. 1. Introduction There are two categories in fibers: natural fibers and synthetic ones. The former category includes cotton, hemp, silk, and wool, and the latter comprises many polymers such as nylon and aromatic polyamide, or socalled aramid. Aramid film was developed by Du Pont (Wilmington, DE) in the early 1960s. Because of its excellent chemical, mechanical, and thermal properties, the film has come to be widely used in various fields. In some aramid products, hydrogen is substituted by other elements such as chlorine to improve their thermal and physical properties, allowing its use for solar cells, magnetic recording tape, special thin belts, electronic parts, and the like.1-4 When the aramid goods are to be disposed of, it is not always easy to decompose them because of their chemically stable properties. As is well-known, combustion operations of such wastes without controlling the temperature have been facing strong public opposition at present because of the possible formation of toxic substances such as dioxins like polychlorinated dibenzo-p-dioxins and polychlorinated dibenzofurans (PCDD/Fs) and polycyclic aromatic hydrocarbons (PAHs). This has given impetus to the development of other safer and more reliable methods for disposing of such products. Accordingly, several methods for this purpose have been proposed and can be classified into two groups: chemical methods and photolysis methods.5-7 Both methods are unique and excellent; however, they need highly controlled conditions in operation as well as facilities with devices and chemical reagents, leading to high cost performance. For this reason, there remains the need to develop an economical method for the disposal and recycling of such wastes. An alternative method has been proposed for decomposing organic polymer wastes containing halogens such as F, Cl, and Br by means of mechanochemical (MC) treatment.8-11 This method is comprised of cogrinding * To whom correspondence should be addressed. Tel. and Fax: 81-22-217-5137. E-mail:
[email protected]. † Sony Company Ltd., Sendai Technology Center. ‡ Tohoku University.
Figure 1. Aromatic amide film constitutional formula.
the waste with inorganic materials such as CaO and water-washing the ground products. The grinding operation results in the formation of hydrocarbons and inorganic chloride, and the washing operation allows the separation of both products from each other. Aramid is known to be a chemically stable material compared with other organic polymers, yet no information has been available for when it is ground with an inorganic material. The main purpose of this work is to provide information on the mechanism of decomposition of aramid, especially the changes in chemical bonding after the MC reaction between aramid and CaO. 2. Experimental Section 2.1. Samples. Chemical reagents used in this experiment were obtained by Wako Pure Chemical Ltd., Japan. A CaO sample was selected for the grinding additive as chlorine absorbent and was prepared by heating Ca(OH)2 at 800 °C for 2 h in an oven. A component of aramid film used in this study is shown in Figure 1. This film is of 4.4 µm thickness and contains about 19.3% by weight of chlorine, which was measured by alkali dissolution and the AgCl weight method. The chlorines are attached to benzene rings. The film sample was mixed with the CaO sample at a 8:1 molar ratio of CaO/Cl. The mixture is used as a starting sample for the grinding. 2.2. Grinding Operation. A planetary mill (Pulversette-7, Fritsch, Germany) was used for grinding the mixture. The mill consists of a pair of pots made of zirconia and a rotating disk. One gram of the mixture (0.61 g of aramid film and 0.39 g of CaO) was put in the pot (45 cm3 inner volume) with seven zirconia balls of 15 mm diameter and subjected to grinding in air at a rotational speed of 700 rpm for various periods of time.
10.1021/ie0300833 CCC: $25.00 © 2003 American Chemical Society Published on Web 09/19/2003
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Figure 2. XRD patterns of the mixed samples (Ca:Cl ) 8:1) ground for different periods of time.
2.3. Characterization. The ground mixtures were characterized by using several methods as follows: X-ray diffractometer analysis (XRD; RAD-B, Rigaku) with Cu KR radiation, scanning electron microscopic observation (SEM; S-4100, 15 kV, Hitachi), particle size analysis (Laser Micronsizer; LMS-30, Seishin Enterprise), thermogravimetric and differential thermal analyses (TG-DTA; TAS-200, Rigaku) at 10 °C/min of heating rate in air, and Raman spectroscopy (Labspec Raman spectrograph, Horiba). The ground mixtures were dispersed in distilled water to dissolve soluble compounds formed during grinding. The filtrate was characterized by ion chromatography (LC10 series, Shimadzu) to determine the ion type and its concentration. The degree to which the MC reaction proceeds was quantitatively evaluated by calculating the percentage of soluble chloride. 3. Experimental Results 3.1. MC Decomposition of Aramid. Figure 2 shows XRD patterns of the mixture ground for different periods of time. The peak intensity of CaO becomes smaller with longer grinding operation. When the grinding is prolonged to 12 h, the peak existence in the pattern practically disappears. In contrast, the main peaks of calcium carbonate (CaCO3) and zirconia (ZrO2) can be observed from the pattern. It is clear that the appearance of ZrO2 comes from the abrasion wear of the zirconia pot and balls. Although the grinding was conducted in air, the pots were sealed during grinding so that there is no basis for stating that the CaO sample absorbs CO2 gas from the outside to form CaCO3. The formation of CaCO3 comes from the MC reaction between CaO and aramid samples. A possible reason for this will be discussed in the later section. Because there are no other peaks observed in the patterns of the ground sample, the chlorine-containing products should be amorphous.
Figure 3 shows the TG (a) and DTA (b) curves of the original aramid film and the mixture samples ground for different periods of time. In the case of the original aramid sample, it is observed that, in the TG curves, weight loss begins around 380 °C and persists continuously until about 640 °C to an end due to thermal decomposition. There exists a correspondingly broad exothermic peak in the temperature range in the DTA curves. In the case of the ground sample, with an increase in grinding time, the weight loss due to thermal decomposition begins at higher and higher temperatures except for the sample ground for 1 h. The higher temperature implies that it is more difficult for the ground samples to combust in air. In other words, the organic compositions after grinding behave more like inorganic ones rather than tending to burn out. Furthermore, compared with the continuous weight loss of the original aramid sample, the ground ones exhibit two sections of weight loss and the latter part is observed at a temperature range up to about 780-880 °C. This is attributed to the decomposition of CaCO3. Consistent with the TG results, in the DTA curves, both exothermic peaks around 500-600 °C due to the combustion of organic composition and endothermic peaks around 800 °C due to the decomposition of inorganic carbonate have been observed. The results from both XRD and TD-DTA analyses have indicated the formation of CaCO3, a similar phenomenon occurring in other MC reactions. As shown in the latter section, organic-bound chlorine is transformed into water-soluble chloride. Accompanying the bonding between Ca and Cl, the bonding between the left partners of oxygen from CaO and carbon from aramid becomes possible. Because CaO is used in extra amounts, free CaO exists after the reaction to form calcium chloride. CaCO3 is formed as a result. It is impossible for all of the carbon composition in the aramid sample to transform into carbonate through the MC solid reaction. The findings on the other part of carbon are discussed on the basis of the Raman analysis results shown in the latter section. As to the 1 h ground sample, both very high and sharp exothermic peaks in the DTA curve and the irregular weight loss in the TG curve suggest a very rapid combustion occurring with the sample. Different from the samples ground for prolonged times (over 2 h) where the MC decomposition has proceeded to some degree and little free aramid or other pure organic sample remains, the 1 h ground sample may contain quite an amount of unreacted aramid sample, more exactly some pure organic composition, which results from the destruction of polyaramid with low molecular weight. This should be the main reason for the rapid combustion when heated in air. Figure 4 shows Raman spectra of the original aramid film and ground samples within different grinding times. The original sample shows a clear aromatic polyamide pattern. However, it is difficult to observe any clear peaks even with the 1 h grinding sample, compared with the original sample. Raman shifts at 1570 and 1320 cm-1 bands are observed, and the peak intensity rises with an increase in the grinding time. Although a little lower than others reported, the two peaks are typical of amorphous carbon. The sharp peak (1570 cm-1) is the graphite band, and the broadened one can be assigned to the finite in-plane domain
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Figure 3. TG (a) and DTA (b) curves of the aramid film and the ground samples for different periods of time.
Figure 4. Raman spectra of the original aromatic amide film and samples with different grinding times.
size,10,11 with other interpretations of the simple D band.12 XRD analysis does not offer evidence about the carbon sample, suggesting that the small domains with limited quantity are below the limits of X-ray detection. Their presence is detected through the 1320 cm-1 peak
of the Raman spectra. Also, the widely broadened pattern implies a wide distribution of the formed domain sizes. The results from Raman spectra clearly show the carbonization of the organic phases with the progress of the decomposition reaction. In fact, the color of the ground samples becomes black with an increase in the grinding time, physically confirming the formation of carbon. Figure 5 shows the SEM photographs of the ground samples for various grinding times. Together with the existence of some big fragments, a rodlike large particle, which may consist of a typical film coated with CaO powders, is observed with the sample ground for 1 h, suggesting that the aramid film has not been destroyed completely by 1 h of grinding. All of the other photographs have shown that the typical morphology of the film becomes unobservable and all of the particles somehow exhibit a morphology of agglomeration. This is a common phenomenon occurring during the dry grinding of inorganic powders. SEM observation has presented physical evidence that the aramid sample has been decomposed after grinding. The cumulative percentages of particle sizes of the grinding samples also show that aramid film changes into small particles by the grinding operation. With an increase in the grinding time, the cumulative curve shifts to the side of small particle size. The average particle size of the grinding samples is as follows: 14.6 µm at 1 h, 6.5 µm at 4 h, and 5.8 µm at 12 h. Figure 6 shows the reaction yield determined by the water-soluble chloride as a function of the grinding time. It is should be mentioned that the reaction progresses smoothly in the early stage of grinding by 2 h. Its yield reaches more than 80% at 2 h, about 97% at 4 h, and almost 100% leaching at 6 h of grinding. This means that the aramid film sample tends to degrade gradually with an increase in the grinding time and almost finishes by MC reaction with CaO to 6 h of grinding. In comparison, in the case of grinding of the aramid film
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Figure 5. SEM photographs of the aramid film and CaO mixture ground for 1, 4, 6, and 12 h.
without CaO addition, only less than 5% water-soluble chlorine was detected even after 6 h of grinding operation. Furthermore, when the mill pot was taken out from a planetary ball mill after grinding, mainly HCl gas emission was detected from the pot. It is obvious that CaO absorbs and reacts with the intermediate of decomposition by MC reaction of the aramid film. The CaO sample exhibits an important role in this MC reaction. It is very interesting to note that the existence of nitrous acid anion (NO2-) was detected by ion chromatography analysis. The calculated percentage of the soluble nitrous anion to the total nitrogen is shown in Figure 7. An increase of the nitrite yield is associated with an increase in the grinding interval. Although the yield of water-soluble nitrite remains as low as about 4% at 12 h of grinding operation, it indicates that the nitrogen in the aramid sample can be oxidized into a nitrite state during the MC reaction. Further investigation is necessary to grasp the transformation route and the state of most of the nitrogen after grinding. 3.2. Discussion. Although not shown here, an experiment on electron spin resonance analysis for the ground sample has indicated the generation of radicals. This leads to charge transfer occurring from the surface of CaO to the site of C-Cl bonds in the organic phase.8,15,16 This allows the disconnection of the C-Cl bonding, leading to MC reaction to reform the chemical
Figure 6. Yield of water-soluble chloride as a function of the grinding time.
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has been detected when an excess amount of CaO is added to the sample. The formation of carbonate and nitrite clearly shows that the grinding facilitates the oxidation of substances already present or formed in the product. 4. Conclusion The film sample of aromatic polyamide (aramid) was mixed with CaO powder, followed by its grinding in ambient air using a planetary ball mill to investigate the MC decomposition of aramid. The experimental results are summarized as follows: (1) MC decomposition of the aramid film sample is achieved by the grinding with CaO. The transformation of organically bound chlorine into inorganic chloride reaches almost 100% by grinding for about 6 h when the molar ratio of Ca to Cl is fixed at 8. (2) CaO plays a significant role not only to catch chlorine removed from the aramid film sample, forming inorganic chloride, but also to exhibit an ability to oxidize the organic compositions to form carbonate and nitrite. (3) The main products are amorphous carbon, CaCl2‚ nH2O, CaCO3. and Ca(NO2)2 when an excess amount of CaO is added. Figure 7. Yield of water-soluble nitrite as a function of the grinding time.
species. One of the new formations is the bond between Cl and Ca, resulting in the formation of CaCl2. At the same time, oxygen removed from the reacted CaO tends to connect with C in the organic phase, forming C-O bonds. This component combines with the excess CaO sample in the product, leading to the formation of CaCO3. On the other hand, the oxygen in CaO does not always react with C but with H, forming O-H bonds. In fact, the hydrogen bonding is formed during dry grinding, and other chemical reactions induce the hydrogen bonding like covalent O-H bonds. When the density of the OH base increases in the organic phase, a dehydration reaction tends to occur, resulting in the condensation of water in the product. The water composition, together with the moisture in ambient air, is absorbed by CaCl2, which has been synthesized in the product, forming its hydrate (CaCl2‚nH2O). However, this hydrate is in an amorphous state, because of the grinding condition, and this is the reason no peaks of this compound have been observed in the XRD patterns, as shown in Figure 2.8,17 After the serial reaction to form new compounds, carbon (C) is left in the product, but it is also in the amorphous state. It is worth mentioning that nitrous acid anion (NO2-) has formed in the product. As described above, the oxygen removed from the CaO by the grinding exhibits a strong ability for oxidation, reacting with C in the organic phase to form CO2. Similar to the oxidation of C, NH groups inside the aramid structure can be also oxidized into a nitrite state, although the percentage of the nitrite state to the total nitrogen in the sample is not so high. Considering the instability of nitrate compounds during grinding, it is difficult to judge whether the nitrite state is the limit. From the MC reaction between organics and CaO, it has been confirmed that CaO reacts with Cl disconnected from the organics. The HCl gas is emitted during the grinding of the organics without CaO, but no emission of the gas
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Received for review January 29, 2003 Revised manuscript received June 26, 2003 Accepted June 26, 2003 IE0300833
