Composition of Nitrogen-Containing Compounds in Oil Obtained from

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Energy & Fuels 2000, 14, 920-928

Composition of Nitrogen-Containing Compounds in Oil Obtained from Acrylonitrile-Butadiene-Styrene Thermal Degradation Mihai Brebu, M. Azhar Uddin, Akinori Muto, and Yusaku Sakata* Department of Applied Chemistry, Okayama University, Tsushima Naka, Okayama 700-8530, Japan

Cornelia Vasile “P. Poni” Institute of Macromolecular Chemistry, 41 A Gr. Ghica Voda Alley, Ro 6600 Iasi, Romania Received February 8, 2000. Revised Manuscript Received April 3, 2000

The thermal degradation of the acrylonitrile-butadiene-styrene copolymer (ABS) was carried out at different temperatures from 360 to 440 °C in static and dynamic atmospheres of nitrogen, using semibatch operation. Nitrogen-containing compounds were found in all three degradation fractions: gases (as NH3 and HCN), oil, and residue. The percentage of the oil fraction increases with the increase of the degradation temperature. At 440 °C 63 wt % of the initial ABS feed was recovered in the oil fraction. The nitrogen (N) concentration of the oil fraction was in the range of 29-40 mg/mL. 4-Phenylbutyronitrile is the main N-containing degradation product (16-19 wt % in oil). N-compounds were also found as aliphatic and aromatic nitriles, amino derivatives, and heterocyclic compounds containing one or two N atoms such as pyridine, pyrimidine, and quinoline. Dynamic atmospheres of nitrogen and the residence time of the products in the reactor affects the oil recovery rate and the distribution of N in the degradation products.

Introduction Due to the limited world’s reserve of coal and crude oil great efforts are being made to find other carbon sources as feedstock materials for the production of fuels. The degradation of waste plastic into fuel represents a sustainable way, for the recovery of the organic content of the polymeric waste, for saving valuable petroleum resources in addition to protecting the environment.1 Municipal waste plastic is a mixture of polymers which contains about 3-5 wt % acrylonitrilebutadiene-styrene copolymer (ABS).2 Unlike other common polymers such as polyethylene, polypropylene, and polystyrene, ABS contains nitrogen (N) in the acrylonitrile units. When ABS is thermally degraded, this N can lead to the formation of ammonia or very toxic hydrogen cyanide in the gas fraction and N-containing compounds in the oil fraction.3 It is wellknown that N is present in coal-derived liquids and shale oil which can lead to the corrosion of engine parts and the formation of very harmful compounds such as HCN or NOx when the oils are used as fuel.4,5 Thermal and catalytic hydrodenitrogenation is the common * To whom the correspondence should be addressed. Fax: +81-86251-8082. E-mail: [email protected]. (1) Sakata, Y.; Uddin, M. A.; Muto, A.; Koizumi, K.; Narazaki, M.; Murata, K.; Kaiji, M. Polym. Recycl. 1996, 2, 309-315. (2) Kajiyama, R. Proceedings of the 1st International Symposium on Feedstock Recycling of Plastics, Sendai, Japan; 1999; pp 9-12. (3) Brebu, M.; Sakata, Y.; Uddin, M. A.; Muto, A.; Murata, K.; Vasile, C. Proceedings of the 1st International Symposium on Feedstock Recycling of Plastics, Sendai, Japan; 1999; pp 123-126.

method used by the petrochemical industry to reduce, as much as possible, the amount of the N-containing compounds in the oil.6 However side reactions can occur during hydrodenitrogenation, resulting in the formation of additional N-containing compounds that are more difficult to eliminate than the original ones.7 N included in five- and six-membered heterocyclic compounds remain as coke inside the refinery catalyst, making its regeneration very difficult.8 We suppose that some of these problems could be reduced in the case of synthetic oil obtained from polymer waste if the degradation is conducted under suitable conditions. Thermal degradation studies on ABS have mainly been performed by thermogravimetry or thermomanometry9-11 and show that the kinetics and mechanism of degradation depend on the chemical structure of the copolymer and the experimental conditions. Various studies12-15 have examined the changes that occur in (4) Radtke, F.; Koeppel, R. A.; Baiker, A. Environ. Sci. Technol. 1995, 29, 2703-2705. (5) O ¨ zdogˇan, S.; Uygur, S.; Egˇrican, N. Energy 1997, 22, 681-692. (6) Cinibulk, J.; Vı´t, Z. Appl. Catal. A 1999, 180, 15-23. (7) Miki, Y.; Sugimoto, Y. Appl. Catal. A 1999, 180, 133-140. (8) Furimsky, E. Catal. Today 1998, 46, 3-12. (9) Suzuki, M.; Wilkie, C. Polym. Degrad. Stab. 1995, 47, 217-221. (10) Yang, M. H. Polym. Test. 2000, 19, 105-110. (11) Vasile, C.; Odochian, L.; Sabliovschi, M.; Vidacov, C. J. Thermal. Anal. 1982, 24, 83-94. (12) Suzuki, M.; Wilkie, C. Polym. Degrad. Stab. 1995, 47, 223228. (13) Klaric´, I.; Roje, U.; Kovacˇic´, T. J. Therm. Anal. 1995, 45, 13731380.

10.1021/ef000018v CCC: $19.00 © 2000 American Chemical Society Published on Web 06/01/2000

Nitrogen-Containing Compounds in Oil

Energy & Fuels, Vol. 14, No. 4, 2000 921

Table 1. Characteristics of ABS and PAN Polymersa elem compn (wt %) sample

C

H

N

Mw (10-4)

Mw/Mn

ABS PAN

84.60 67.84

7.80 5.65

6.65 25.54

17.2 37.7

7.2 2.2

a M , weight-average molecular weight; M , number-average w n molecular weight.

the thermal properties of materials when ABS is grafted or blended with other polymers. In addition there have been studies concerning the thermal behavior of polyacrylonitrile16,17 or styrene-acrylonitrile binary copolymers.18,19 Day and co-workers20 present some results from ABS thermal degradation in a pyrolysis/gas chromatography/mass spectrometry system, alone and together with poly(vinyl chloride) (PVC). No study was made with special interest in using ABS degradation products as fuel oil. This paper presents a characterization of the Ncontaining products resulting from the degradation of ABS under different thermal conditions as a first step to finding the most suitable degradation conditions to obtain a good quality fuel oil from plastic waste. ABS was degraded by a semibatch operation, and the products were collected and analyzed by different gas chromatographic methods. The main interest was to determine the nature of the N-containing compounds, especially those in the degradation oil, and to understand how the product distribution is affected by the degradation conditions. Experimental Section (1) Materials. Acrylonitrile-butadiene-styrene powder copolymer (ABS) containing 19-22% acrylonitrile, 37-39% butadiene, and 30-32% polystyrene units was obtained from Aldrich Chemical Co. The N in the ABS comes from acrylonitrile units, so we compared the degradation of ABS with that of pure polyacrylonitrile (PAN), also obtained from Aldrich. The amounts of C, N, and H in the polymer samples were determined by elemental analysis and the molecular weights by GPC method. The characteristics of these two N-containing polymers are presented in Table 1. (2) Decomposition Procedure. The degradation experiments were performed in a glass reactor, under atmospheric pressure, using semibatch operation (Figure 1a). The reactor has an internal diameter of 30 mm and a total length of 350 mm. The top 70 mm of the reactor protrudes out of the furnace so a part of the heavy decomposition products condenses and returns to the reactor for further decomposition. The degradation oil was condensed in a condenser and collected in a graduated cylinder that allowed for the determination of the accumulation rate. Gaseous products were passed through a flask with water to trap ammonia and hydrogen cyanide before non-water-soluble gases were collected in a Teflon bag. A 10 g amount of polymer was degraded in each experiment, using different temperature programs (Figure 1b) and atmo(14) Klaric´, I.; Roje, U.; Bravar, M. J. Appl. Polym. Sci. 1996, 61, 1123-1129. (15) Klaric´, I.; Roje, U.; Stipanelov, N. J. Appl. Polym. Sci. 1999, 71, 833-839. (16) Vasile, C. St. Cerc. Chim. 1973, 21, 81-104. (17) Xue, T. J.; McKinney, M. A.; Wilkie, C. Polym. Degrad. Stab. 1997, 58, 193-202. (18) Kressler, J.; Rudolf, B.; Shimomai, K.; Ougizawa, T.; Inoue, T. Macromol. Rapid Commun. 1995, 16, 631-636. (19) Xue, T. J.; Wilkie, C. Polym. Degrad. Stab. 1997, 56, 109-113. (20) Day, M.; Cooney, J. D.; Touchette-Barrette, C.; Sheehan, S. E. J. Anal. Appl. Pyrol. 1999, 52, 199-224.

spheres. Prior to degradation the reactor was flushed with N2 at a flow rate of 20 cm3/min while the temperature of the furnace was maintained at 120 °C for 1 h in order to eliminate adsorbed water. The zero time for the experiment (marked × in Figure 1b) was taken when the furnace was heated again at 3 °C/min from 120 °C to the final degradation temperatures, which were fixed at 360-440 °C. The total reaction time was 540 min in all cases. The degradation conditions are summarized in Table 2. In “mode I” degradation occurs in a static N2 atmosphere, after the N2 flow was cut off at time zero. In “mode II” a dynamic N2 atmosphere of 20 cm3/min was maintained in the reactor during the entire process. In “mode III” the degradation experiment used a dynamic N2 atmosphere and an isothermal hold of 300 °C for 1 h before heating to the final temperature of 400 °C. In “mode IV”, the reactor was completely immersed in the furnace and the top of the reactor was insulated with glass wool. The temperature was set to 440 °C, and the N2 dynamic atmosphere was maintained in order to obtain a short residence time (SRT) of the products in the reactor. (3) Analysis Methods. The gaseous products were analyzed with a gas chromatograph-thermal conductivity detector instrument (TCD; YANACO G180; column, PORAPAK QS; 2.5 m; isothermal condition, 100 °C). The quantitative analysis of the degradation compounds in oils was performed with a gas chromatograph-flame ionized detector instrument (FID; YANACO G6800; column, 100% methyl silicone; 50 m × 0.25 mm × 0.25 µm; temperature program, 40 °C (hold 15 min) f 280 °C (rate 5 °C/min; hold 37 min). The identification of the main products in the degradation oils was performed with a gas chromatograph-mass selective detector instrument (MSD; HP 5973; column, HP-1; cross-linked methyl siloxane, 25 m × 0.32 mm × 0.17 µm; temperature program, 32 °C (hold 2 min) f 50 °C (rate 1 °C/min) f 300 °C (rate 5 °C/min). The distribution of nitrogen in oils was determined with a gas chromatograph-atomic emission detector instrument (AED; HP G2350A; column, HP-1; cross-linked methyl siloxane; 25 m × 0.32 mm × 0.17 µm; temperature program, 40 °C (hold 10 min) f 50 °C (rate 2 °C/min) f 70 °C (rate 1 °C/min) f 100 °C (rate 3 °C/min) f 120 °C (rate 1 °C/min) f 300 °C (rate 10 °C/min; hold 2 min). Nitrobenzene was used as the internal standard for the quantitative determination of carbon and nitrogen in the GC-AED analysis. The composition of the degradation oil was characterized using C-NP-grams21 (C stands for carbon and NP from normal paraffin) and N-NP-grams22 (N stands for nitrogen). These curves were obtained by plotting the weight percent of the products and the N amount respectively in the degradation oil versus the carbon number of the normal paraffin having equivalent boiling points. The equivalence with the normal paraffin was made by comparing the retention times from GC analysis using a nonpolar column. In these conditions hydrocarbons appear in the order of increasing boiling points. However, polar compounds such as nitriles or amines exit the column earlier than the corresponding hydrocarbons having similar boiling points; therefore, their carbon number in an NP-gram will be lower than the real one. This difference in carbon number for the polar compounds will not affect the relative comparison of the samples. Ammonia absorbed in a water trap was determined by titration with hydrochloric acid, and hydrogen cyanide also absorbed in water was determined by means of an ion meter (WTW inoLab pH/ION level 2) using a cyanide ion selective electrode. The polymers and residues of the degradation were analyzed by infrared spectroscopy (FT-IR; Nicolet Prote´ge´ 460) in solid phase, after mixing with potassium bromide, in the diffuse (21) Murata, K.; Makino, M. Nippon Kagaku Kaishi 1975, 1, 192200. (22) Shiraga, Y.; Uddin, M. A.; Muto, A.; Narazaki, M.; Sakata, Y.; Murata, K. Proceedings of the 1998 International Symposium on Advanced Energy Technology, Sapporo, Japan; 1998; pp 185-192.

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Figure 1. Schematic diagram of experimental setup (a) and the temperature programs (b).

Figure 2. Cumulative volume of liquid product obtained from ABS thermal degradation.

Figure 3. Initial rate of oil accumulation from ABS and PAN degradation.

Table 2. Experimental Conditions for ABS Degradation

initial rate of oil accumulation was calculated over the next 30 min. These data are presented in Figure 3. In Table 3 the material balance and the global characteristics of the final oil obtained from the ABS and PAN degradations in a static N2 atmosphere (mode I) are presented. In the thermal degradation of ABS the shape of the curve of the cumulative oil volume is specific for each degradation temperature (Figure 2). The initial rate of oil accumulation has an exponential dependence with the temperature. The oil accumulation rate for PAN degraded at 400 °C occurs very slowly compared to ABS (Figure 3) and at an almost constant rate during the entire process. Based upon the data presented in Figure 3 and Table 3, it can be seen that ABS copolymer decomposes faster than PAN and gives a higher quantity of oil. From the material balance of 10 g of ABS degradation one can see an increase in the oil and gas amounts with increasing temperature to the detriment of the solid residue. The degradation oil obtained at 360 °C is a light, transparent yellow liquid, but the oil becomes cloudy with darker light brown color when the degradation occurs at high temperatures. The density of the oil increases with temperature, indicating a change in oil composition. The concentration of N in ABS degradation oil is in the range of 29-40 mg/mL, having the highest value

degradation N2 mode atmosphere I II III

static dynamic dynamic

IV

dynamic

final temp (°C)

particularities

from 360 to 440 from 400 to 420 400 pretreatment for 1 h at 300 °C 440 reactor completely in the furnace (short residence time)

reflectance mode. Identification of the chemical bonds was based upon the adsorption bands, according to an IR spectral database.23

Results and Discusions (1) Degradation in a Static Atmosphere (Mode I). (1.1.) Degradation Behavior and Global Characterization of Products for ABS and PAN. The products of ABS degradation were classified in three groups: gas, oil, and degradation residue. Figure 2 shows the cumulative volume of the oil fraction obtained from ABS thermal degradation. The oil starts to accumulate in the graduated cylinder after about 80 min from the time zero of the experiment. The (23) Balaban, A. T.; Banciu, M.; Pogani, I. Applications of Physical Methods in Organic Chemistry; Stiintifica si Enciclopedica: Bucharest, 1983; pp 20-36.



Table 3. Material Balance for ABS and PAN Degradation in Static N2 Atmosphere (Mode I) product yield (wt %)

a

gas

(G)a

sample

temp (°C)

oil (L)

residue (R)

oil density (g/mL)

N in oil (mg/mL)

ABS ABS ABS ABS ABS

360 380 400 420 440

3.4 4.2 7.2 6.5 7.8

23.7 38.9 49.5 56.0 63.5

72.9 56.9 43.3 37.5 28.7

0.8495 0.8683 0.8684 0.8750 0.8819

35.3 38.3 39.9 31.2 29.3

PAN

400

17.5

4.3

78.2

0.9149

183.6

G ) 100 - (L + R). Table 4. Balance of N in Different Thermal Degradation Products (Mode I) N in degradation products (wt % of initial N content of polymer) gases (G)

oil (L)

sample

temp (°C)

N in 10 g of sample (mg)

NH3

HCN

HCN

Organic N

residue (R)a

ABS ABS ABS ABS ABS

360 380 400 420 440

665 665 665 665 665

0.06 0.16 3.52 1.58 3.77

0.05 0.11 0.59 0.54 1.49

1.35 2.11 1.92 1.75 1.77

13.48 23.68 32.32 28.25 29.92

85.06 73.94 61.65 67.88 63.05

PAN

400

2554

27.76

5.10

1.07

2.31

63.76

a

residue ) 100 - (gases + oil). R ) 100 - (G + L).

at 400 °C. Table 4 shows the amount of N in terms of the percentage from the initial N in the sample that goes in different degradation products. The percentage of N that goes to oil and gases increases with the degradation temperature from 360 to 400 °C and remains almost constant for temperatures between 400 and 440 °C. This is explained by the cyclization of acrylonitrile units that occurs at low temperatures with elimination of NH3 and HCN and formation of N-containing compounds in oil, while above 400 °C the reaction of backbone scission producing N-free hydrocarbons becomes prevalent. Acrylonitrile units represent only 19-22 wt % in ABS copolymer, so we compare the results with PAN degradation where the acrylonitrile thermal behavior is not affected by the presence of other components. PAN degradation at 400 °C gives only 4.3 wt % oil, but the N concentration in oil is 183.6 mg/mL (last column in Table 3), which means 20 wt % N, which is a very high value. However, organic N represents only 2.31% (column 7 in Table 4) from the initial amount of N in polyacrylonitrile. From ABS degradation at the same temperature (400 °C) a higher quantity of 32.32% of the N goes as organic N in the oil, probably due to the reactions of the backbone scission producing a higher quantity of liquid products and a low quantity of solid residue as well as the possible interaction that appears between the acrylonitrile units and the butadiene and styrene comonomers. The total amount of N that goes in NH3 and HCN (sum of columns 4-6 in Table 4) represents less than 7% from the initial amount of N in ABS. One can say that the ABS degradation mechanism is different with respect to that of PAN, where 34% of the N amount is converted to NH3 and HCN, or other polymers with the side group elimination mechanism such as PVC, where the chlorine could be 99% eliminated from the macromolecular chain as gaseous HCl.24 This is additional (24) Miranda, R.; Pakdel, H.; Roy, C.; Darmstadt, H.; Vasile, C. Polym. Degrad. Stab. 1999, 66, 107-125.

support for possible chain scission reactions in ABS thermal degradation. More than 60% of the initial N remains in the solid residue (last column in Table 4), both from ABS and PAN degradation, which is in good agreement with results reported in other papers.11,17 (1.2) Composition of ABS Degradation Products. In Table 5 are listed all the compounds identified in ABS thermal degradation products by means of GC-TCD and GC-MS analyses. The compounds are classified in three categories: (a) hydrocarbons in gases, (b) hydrocarbons in oil, and (c) N-containing compounds. Discussion on their amounts and distribution in the products are made in the following sections. (1.3) Hydrocarbon Compounds. Besides HCN and NH3, the gaseous products contain a high quantity of hydrocarbons such as methane, ethylene, ethane, propylene, propane, unsaturated and saturated C4 and C5 hydrocarbons (listed in Table 5, column a), and traces of benzene, toluene, ethylbenzene, and styrene. Figure 4 presents the composition of the main gaseous hydrocarbons obtained from ABS thermal degradation as determined by GC-TCD. For the degradation performed at 360 °C propylene represents 58 vol % of the total gas fraction, while at higher temperature methane and ethane become the main products and the C5 hydrocarbons are not detected. Therefore the amount of light compounds increases at higher decomposition temperatures. The C-NP-gram presented in Figure 5a shows the distribution of the products (weight percent in oil) with the carbon number of the normal paraffin having equivalent boiling points. The amount (mg) of the main hydrocarbons identified in the degradation oil obtained from ABS is presented in Figure 5b. The hydrocarbons provided in Figure 5b are grouped in a way that corresponds to their equivalent carbon number in the C-NPgram (Figure 5a). For example the compounds marked from d to h correspond to the n-C8 to n-C10 range. The compounds obtained from ABS degradation are distributed in oil in the range of boiling points of n-C5

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Brebu et al.

Table 5. Compounds Identified in ABS Thermal Degradation Products (a) hydrocarbons in gases (Figure 4) methane ethylene ethane propylene propane C4 isomers C5 isomers

a

(b) hydrocarbons in oil (Figure 5b) code name a b c d e f g h i

butylene cyclohexane benzene toluene ethylbenzene styrene isopropylbenzene R-methylstyrene 1,3-diphenylpropane

code a b c d e f g h i j

(c) N-containing compds (Figure 6b) name formula ammonia hydrogen cyanide acetonitrile acrylonitrile propionitrile metacrylonitrile isobutyronitrile crotonitrile phenylacetonitrile 2-phenylpropylonitrile 4-phenylbutyronitrile methylquinoline or R-naphthylamine

NH3 HCN CH3CN CH2dCHCN CH3CH2CN CH2dC(CH3)CN CH3CH(CH3)CN CH3CHdCHCN PhCH2CN PhCH(CH3)CN Ph(CH2)3CN

dimethylquinoline isomersa

e.g.,

l

N-benzylpyrrole

m

phenylethylamine

n

N-benzylaniline

o

hexadecanonitrile 2-amino-4,6-dimethylpyridine or 2-ethyl-3-methylpyrazine

C15H31CN

q

octadecanonitrile

C17H35CN

Not determined quantitatively.

Figure 4. Composition of hydrocarbon gaseous products obtained from ABS thermal degradation at different temperatures.

to n-C25 (bp, -0.5 to +405.1 °C), with a main peak at n-C9 (30-49 wt %) and two other peaks at n-C6 (2-6 wt %) and n-C13 (4-19 wt %), as shown in Figure 5a. With the increase of the degradation temperature, the peak at n-C9 decreases and the one at n-C13 increases. It is also noted that the compounds from n-C15 to n-C18 appear to increase with degradation temperature, which could explain the increase in density of the oils.

The compounds determined in the n-C8 to n-C9 range are only hydrocarbons with no N-containing compounds. The most important ones are toluene, ethylbenzene, styrene, cumene, and R-methylstyrene (listed in Table 5, column b, their absolute amount presented in Figure 5b). Most of these compounds usually originate from polystyrene units, although some could come from butadiene units. We identified toluene, styrene, and R-methylstyrene in the degradation oil of an acrylonitryle-butadiene copolymer but not in the oil obtained from PAN degradation. Increasing the degradation temperature increases the amount of 1,3-diphenylpropane, the main hydrocarbon in the n-C17 range, while toluene and ethylbenzene decrease. (1.4) Nitrogen-Containing Compounds. The variation in the quantity of ammonia, hydrogen cyanide, and other N-containing compounds in degradation products with reaction conditions is discussed above. In Figure 6a the N-NP-gram is expressed in terms of the N concentration (milligrams per milliliter in oil) versus the carbon number, while the major N-containing compounds provided in Figure 6b are grouped corresponding to their equivalent carbon number. The amount



Figure 5. C-NP-gram (a) and the amount of the main hydrocarbons (b) in ABS degradation oil.

of compound j is detailed inside because it is 4-6 times higher than the other compounds. Figure 7 shows the C-NP-gram and N-NP-gram of PAN degradation oil obtained at 400 °C for comparison purposes. The N-containing compounds found in the oil from the degradation of ABS are distributed mainly in the range of boiling points of n-C5 to n-C14 (-0.5 to +252.5 °C), with peaks at n-C5 to n-C6, n-C10, and n-C13, but small amounts can be detected up to n-C21 (Figure 6a). Increasing the degradation temperature decreases the n-C5 and n-C6 compounds while increasing those of a higher molecular weight. A significant amount of hydrogen cyanide (about 20 mg) is dissolved in oil (compound a in Figure 6b and Table 5, column c). The N-containing compounds in the n-C5 to n-C6 carbon number range were identified as aliphatic saturated and unsaturated nitriles with one to three carbon atoms (compounds b-g listed in Table 5, column c, with their amounts presented in Figure 6b). These compounds are typical of the decomposition products of acrylonitrile units in ABS and have been identified to be the main degradation products obtained from PAN (maximum at n-C5 to n-C6 in Figure 7). The compounds corresponding to n-C12 and n-C13 are aromatic nitriles (compounds h-j in Table 5, column c, and Figure 6b). 4-Phenylbutyronitrile is the main Ncontaining product, and its amount significantly increases with the temperature.

For carbon numbers higher than 14 the N-containing compounds have complex structures: aromatic amines or products with one or two N atoms included in heterocyclic compounds, such as pyridine, pyrimidine, pyrrole, and quinoline derivatives. Methylquinoline and dimethylquinoline isomers were also identified by GC-MS, but it was difficult to determine their amounts in the oil; consequently, they are not presented in Figure 6b, and they have not been given a code in Table 5. Two or three peaks were detected at n-C10 in GCAED, but a clear identification of their structures was difficult. However, from GC-MS analysis it would appear that they correspond to aniline, pyridine, or amino derivatives. (1.5) Solid Residue. Figure 8 shows the FT-IR spectra of the initial ABS polymer and the degradation residues. The adsorption bands in the FT-IR spectra of the ABS degradation residues become wide and more difficult to interpret as the degradation temperature increases. This is due to the increase in carbon content caused by the advanced degradation. The FT-IR spectrum of the ABS polymer exhibits a strong and sharp adsorption band at 2237 cm-1 corresponding to the ν(CN) vibration associated with the acrylonitrile unit. In the spectra of the degradation residues, this band is slightly shifted to higher frequency and decreases in intensity with increase in the degrada-

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Figure 6. N-NP-gram (a) and the amount of the main nitrogen-containing products (b) in ABS degradation oil.

Figure 8. FT-IR spectra of ABS polymer and residues obtained from ABS degradation at different temperatures.

Figure 7. C-NP-gram and N-NP-gram of PAN degradation oil at 400 °C.

tion temperature. A shoulder corresponding to aromatic nitriles appears at 2223 cm-1 that becomes more important and finally remains the only band in the region for the spectra of the residue obtained at 440 °C, showing that the -CN groups in the residue are bonded mainly to aromatic structures. This suggests that cyclization occurs during ABS degradation similar to the polyacrylonitryle thermal degradation process utilized for carbon fiber production. The adsorption band at 3380 cm-1 with a shoulder at 3480 cm-1 corresponds to

symmetric and antisymmetric ν(NH2) vibration. Meanwhile the weak band at 2725 cm-1 corresponds to ν(C-H) vibration in the CH groups substituted with N. All these bands are present in the spectra of the residues only and suggest the formation of amino derivatives, especially when the degradation occurs at low temperatures. The FT-IR spectrum of the ABS exhibit sharp adsorption bands at 1600, 1500, and 1450 cm-1 and an harmonic of bands at 1660-2000 cm-1, all corresponding to vibration of aromatic rings from styrene units. In the FT-IR spectra of residues these bands becomes difficult to assign with increasing the degradation temperature, except the band at 1600 cm-1, which becomes wider but remains distinct even after degradation at 440 °C. The band at 1600 cm-1 in the residues



Table 6. Material Balance for ABS Degradation in Dynamic N2 Atmosphere product yield (wt %) degradation mode

temp (°C)

II II III IV a

400 420 400 440

gas

(G)a

9.6 10.3 10.1 9

oil (L)

residue (R)

oil density (g/mL)

N in oil (mg/mL)

37.8 47.9 34.0 73.4

52.6 41.8 55.9 17.6

0.8591 0.8789 0.8718 0.9062

30.3 29.3 27.3 24.7

G ) 100 - (L + R). Table 7. Balance of N in Different Degradation Products (Dynamic N2 Atmosphere) N in degradation products (wt % of initial N content of polymer) gases (G)

degradation mode

temp (°C)

N in 10 g of sample (mg)

NH3

HCN

HCN

organic N

residue (R)a

400 420 400 440

665 665 665 665

10.03 11.02 10.12 7.73

3.62 3.70 2.54 1.91

0.46 0.52 0.36 0.81

19.6 23.48 15.64 29.28

66.29 61.28 71.34 60.27

II II III IV a

oil (L)

R ) 100 - (G + L).

Figure 9. C-NP-gram of oil obtained from ABS degradation in dynamic N2 atmosphere.

Figure 10. N-NP-gram of oil obtained from ABS degradation in dynamic N2 atmosphere.

can be assigned to the ν(CdN) vibration, which confirms the formation of a polyiminic structure that appears to be associated with the cyclization of acrylonitrile units, as observed in the PAN degradation.17 (2) Degradation in Dynamic Nitrogen Atmosphere (Modes II, III, and IV). The material balance and the balance of N in the degradation products when ABS degradation occurs in dynamic N2 atmosphere are presented in Table 6 and Table 7. The C-NP-gram and N-NP-gram of the oil obtained in these conditions are presented in Figure 9 and Figure 10, respectively. When a N2 flow is used during the degradation (mode II), the product distribution ratio changes, favoring the formation of the gases and residues (Table 6). At the same time the amounts of ammonia and hydrogen cyanide produced are significantly increased (Table 7) in comparison to mode I (Table 4). The amounts of the main hydrocarbons produced (listed in Table 5, column b) remain essentially unchanged; however, the concentrations of all the N-containing compounds in the oil decrease (Figure 10), especially the amount of the

dissolved HCN and light aliphatic nitriles from n-C5 that might be removed from the reactor by the N2 flow. When 1 h pretreatment at 300 °C was used (mode III), the smallest quantity of liquid product was obtained (34 wt %), having the lowest concentration of N. While the pretreatment had no influence on the amount of ammonia in the gas, the N-containing compounds present in the oil significantly decreased except for the high molecular weight species which showed an increase (mainly those corresponding to n-C17 to n-C21). The main part of the initial N in the ABS was found to remain in the solid residue (71.34%, last column in Table 7). This suggests a promotion of the cyclization reactions due to the pretreatment. Experimental conditions for degradation in mode IV (reactor totally introduced in furnace, dynamic N2 atmosphere, and high temperature of 440 °C) ensure a short residence time (SRT) of the products in the furnace. Under these conditions the highest percentage of recovered oil (73.4 wt %) was obtained with the oil having a greenish yellow color and a higher density compared with all the other cases. This implies that fast

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removal of the degradation products from the reactor favors the scission of the ABS macromolecular chain. The residence time of the products in the reactor is too short for the secondary reaction of degradation to occur, and the quality of the oil is not good. Under mode IV conditions the maximum in the C-NP-gram (Figure 9) occurs at n-C9 with a 7.9 wt % increase in comparison with degradation at 440 °C with mode I (static N2 atmosphere), showing an increase of the amount of hydrocarbons in the liquid products. This is the reason for increasing the oil amount by mode IV (and as a result decreasing the N concentration) because the percentage of organic N that goes to oil is the same for the two cases (ca. 30 wt %, as shown in Table 4, column 7, for mode I and Table 7, column 7, for mode IV). The oil obtained by mode IV has significantly increased the amounts of the high molecular weight N-containing compounds, especially those listed from n to q in Table 5, column c. The presence of these compounds could explain the green color of the oil. The amount of ammonia is also increased (2 times), but the amount of hydrogen cyanide decreases, especially the amount dissolved in the oil.

Brebu et al.

Conclusions The degradation of the acrylonitrile-butadienestyrene (ABS) copolymer by a semibatch operation at temperatures between 400 and 440 °C gives 50-63 wt % oil with 29-40 mg/mL concentration of N. The temperature of degradation significantly affects the rate of evolution and the amount and the quality of the degradation oil. Using a N2 dynamic atmosphere or changing the residence time of the products in the reactor also affects the products of ABS degradation. Nine main hydrocarbons and seventeen N-containing compounds were identified in the degradation oil. The N is present mainly as aliphatic and aromatic nitriles. Heterocyclic compounds with one or two N atoms were identified in small amounts. More than 50 wt % of the degradation oils consist of hydrocarbons such as toluene, ethylbenzene, styrene, isopropylbenzene, and R-methylstyrene, and as such it represents a possible hydrocarbon source or fuel provided the concentration of N-containing compounds can be decreased to an acceptable level. EF000018V

Composition of Nitrogen-Containing Compounds in Oil Obtained from

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