폴리프로필렌과 폴리스티렌 혼합 플라스틱의 촉매 분해

윤직현, 김종렬, 반종호, 박대원

부산대학교 화학공학과

Catalytic Degradation of Plastics Mixtures of Polypropylene and Polystyrene

J.H. Yoon, J.R. Kim, J.H. Van, D.W. Park

Department of Chemical Engineering, Pusan National University

Introduction

The total amount of plastic wastes generated by our society is growing rapidly. The low biodegradability of plastic creates a serious environmental problem that is directing the governments to propose a solution to solve the problem based on source reduction, re-use and recycling [1]. Thermal cracking and hydrocracking have been investigated at laboratory and pilot plant level and some semicommercial installations have been built for the treatment of mixed plastic wastes, even those containing chlorine [2]. However, thermal cracking yields low value unstable hydrocarbons within a very broad range of boiling points. Catalytic cracking operates at lower temperatures than thermal cracking and produces better quality products with the corresponding economical improvement.

A large number of laboratory studies have been conducted for the direct catalytic cracking of different type of plastics. To date, the bulk of the research has focused on elucidating the mechanism of degradation for pure polymers. Most waste streams, however, contain a mixture of polymers which are costly to segregate. Two polymers, polypropylene (PP) and polystyrene (PS) are major sources of polymers in municipal solid wastes. In addition, a large variety of catalysts studied previously can be unrealistic from the point of view of practical use due to the cost of manufacturing and the high sensitivity of the process to the cost of the catalyst. In our previous works [3, 4], we carried out the degradation of PP and PS using natural clinoptilolite catalyst and it was proved to be a good candidate for the polyolefin degradation. For these reasons, mixtures of PP and PS were selected to study the performance of the natural clinoptilolite catalyst in their degradation. The special focus of this study will be on the interaction between two polymers, effect of acid treatment and degradation temperature on the distribution of hydrocarbon products.

Experimental

Polypropylene (PP), in powder form, is an isotactic sample obtained by Honam Petrochemical Co. (in Yeacheon, Korea) as FC-150, with an average molecular weight (Mw) of 329700. Polystyrene (PS), in powder form, was supplied by LG Chemical Co. (Grade 50IS, Mn=98000-99000, Melt Index=7.5g/10min, density=1.03 g/cm3). The plastic samples of 60-150 mesh size were used for this study. Several types of solid acid catalysts such as silica-alumina (SA, Aldrich), ZSM-5 (RC-Penta) and natural clinoptilolite zeolite (NZ, occurring in Pohang area of Korea) were evaluated through the degradation experiments of PP, PS and their mixture (PP+PS). ZSM-5 and NZ were ion-exchanged three consecutive times with 1M NH4Cl solution for 20h. The zeolites exchanged with NH4+ were dried at 110 ℃ for 6h, and then calcined in air at 400 ℃ to obtain the proton(H+)-exchanged zeolites like HZSM-5 and HNZ. The catalytic degradation of the plastics was carried out in a semi-batch reactor where nitrogen is continuously passed with a flow rate of 30 mL/min. A mixture of 3.0 g of the plastics and 0.3 g of the catalyst was loaded inside a pyrex vessel of 30 mL and heated at a rate of 30 ℃/min up to the desired temperature. The distillate from the reactor was collected in a cold trap over a period of 2h.

The degradation of the plastic gave off gases, liquids and residues. The residue means the carbonaceous compounds remaining in the reactor and deposited on the wall of the reactor. The amount of coke deposit on the catalyst was calculated by measuring the desorbed amount of carbon dioxide during temperature programmed oxidation of used catalysts. The gases were  analyzed by an on-line GC (HP 5890) with a Porapak Q column. The condensed liquid samples  were analyzed by GC-MS (Micromass Co., Auto spec.) with a capillary column (HP-5MS). The physical properties and the composition of the liquids were also measured by a PONA (Paraffins-Olefins-Naphthenes-Aromatics) analyzer (DHA 2520).

Results and Discussion

In order to check possible interactions between PP and PS, a 50 wt.% of each polymer was mixed for the thermogravimetric analysis. Experimentally obtained TG and DTG curves for the mixture and the sum of the individual components are presented in Fig. 1. The curves representing the sum of the individual components are derived by:

(1-□)cal=¤Yi(1-□)i	(1)

In Eq. (1), Yi is the mass fraction of component i, (1-□)i is the normalized mass loss for component i as found from single component experiment. The rate of weight loss for the sum of single components is derived in the same way as for the weight loss.

The thermal degradation of PP+PS mixtures are previously studied to understand the interaction between two polymers. It is reported that PP begins to degrade at lower temperatures in the presence of PS, suggesting an attack of PP by polystyryl radicals. Murata and Akimoto [5] also studied the PP+PS mixture by TG and GC analysis. They found that when PS is present in the mixture, PP is destabilized. However, there are few reports on the interaction of PP and PS for the catalytic degradation of the mixture.

In the case of PP+PS mixture (Fig. 1), the experimental TG/DTG curve is very different with the calculated non-interaction curve. After the first 15 % weight loss, the PP degradation shifts to a higher temperature compared with the pure PP degradation. However, after 60 % weight loss, PS degradation is accelerated more than the calculated one. The maximum degradation rate of the mixture in DTG curve is at 400 ℃, which is lowered by about 20 ℃ compared to that for PS. Catalytic degradation of PP and PS is known to be of carbenium nature. From our experimental findings and literature results, one can suggest that the mixing effect on the catalytic degradation rate comes from the intermolecular transfer of carbenium ion between the degraded fragments of PP and PS. The carbenium ion of PP can accelerate the degradation of PS.

Table 1 shows the mass balance of degradation products for the mixture of PP and PS at 400 ℃ for 2 h. The arithmetic average value of each product are also presented in this table. The main degradation product was liquid oil, however, a little more residue was formed than that of calculated value in the degradation of the mixture plastics. All the catalysts produced higher amount of gases and lower amount of residues than thermal degradation did. It is noteworthy that the gas production of HZSM-5 was largely decreased compare to the calculated value. It is well known that the degraded fragments of PS has more difficulties to penetrate into the active sites of catalysts due to their structure containing aromatic group. This will inhibit the further cracking of PP degradation fragment and hence reduce the gas fraction in the case of HZSM-5.

Table 2 lists the product distribution in hydrocarbon type. The aromatic amounts in the mixture plastics were higher than that expected from the individual components. The olefin amount, however, shows much lower values than calculated ones. This means that the conversion of olefinic intermediates from PP degradation was promoted by the presence of aromatic fragments of PS degradation.

The effect of acid treatment on the degradation performance was studied using the natural zeolite catalysts and HZSM-5; the Na-form (Na-NZ, ZSM-5) and the protonated form (HNZ, HZSM-5). Since the aromatics are major products, their compositions are compared in Table 3. One can see that ethylbenzene selectivity increases (from 15.1 to 21.3 %) and styrene selectivity decreases (from 53.7 to 45.2 %) from NZ to HNZ. From ZSM-5 to HZSM-5, one can also find out a decrease of styrene selectivity (from 67.3 to 60.0 %) and an increase of ethylbenzene selectivity (from 5.9 to 9.8 %). This result indicates that the hydrogenation of degraded styrene can occur more easily in the protonated catalysts and that it leads to the increased production of  ethylbenzene. The acid catalyzed cracking of PS is of carbenium nature. The most likely reaction pathway involves the attack of proton associated with a Br□nsted acid site to the aromatic rings of PS, due to the reactivity of its side phenyl groups towards electrophilic reagents. The resulting carbenium ion may undergo □-scission followed by a hydrogen transfer.

However, Nanbu et al. [6] reported that the protonated polymer backbone could also proceed through cross-linking reactions among adjacent polymeric chains or even inside the same polymer. Therefore, the cross-linking reactions and cracking reaction are competitive in PP+PS degradation. When the cross-linking reactions are accelerated, the competing catalytic cracking may occur to a smaller extent.

Acknowledgement

The authors wish to acknowledge the financial support of the Korea Research Foundation (2000-1-30700-008-3).

References

1. Y. Uemichi, Catalysis (Japan) 37 (1995) 286.

2. S. Shelley, K. Fouhy, S Moore, Chem. Eng. 99 (1992) 30.

3. S.Y. Lee, J.H. Yoon, J.R. Kim, D.W. Park, Polym. Degrad. Stab. in press (2001).

4. E.Y. Hwang, J.R. Kim, J.K. Choi, H.C. Woo, D.W. Park, J. Anal. Appl. Pyrol. in press (2001).

5. K. Murata, M. Akimoto, Chem. Ind. Chem. (Japan) 6 (1979) 674.

6. H. Nambu, Y. Sakuma, Y. Ishihara, T. Takesue, T. Ikemura, Polym. Degrad. Stab. 19 (1987) 61

Table 1. Mass balance in the degradation of the mixture of PP and PS at 400℃ for 2h

Catalyst	Gases		Liquid		Residues		Cokes
	Exp.	Cal.	Exp.	Cal.	Exp.	Cal.	Exp.	Cal.
Thermal	18.3	11.1	67.1	78.5	14.6	10.4	-	-
SA	24.1	15.9	65.5	79.2	8.0	2.6	2.4	2.3
HNZ	23.5	17.3	64.5	77.9	9.7	2.5	2.3	2.3
HZSM-5	28.3	40.3	61.4	54.2	8.1	3.2	2.2	2.3

Table 2. Distribution of liquid products (wt..%) in the degradation of mixture of PP and PS at 400℃ for 2h

Catalyst	n-Paraffins		i-Paraffins		Olefins		Naphthenes		Aromatics		Oligomers
	Exp.	Cal.	Exp.	Cal.	Exp.	Cal.	Exp.	Cal.	Exp.	Cal.	Exp.	Cal.
SA	0.85	0.52	6.0	11.4	12.6	23.1	3.0	5.2	77.5	59.6	trace
HNZ	0.48	0.32	4.6	9.7	8.7	25.5	6.8	5.1	79.4	59.4	trace
HZSM-5	0.44	1.36	5.2	4.9	7.0	18.4	4.0	3.6	83.4	71.8	trace

Table 3. Composition (wt.%) of aromatics (C6-C13) for the degradation of PP+PS at 400℃ for 2h

Aromatics	NZ	HNZ	ZSM-5	HZSM-5
Benzene	0.53	1.12	0.55	1.19
Toluene	7.98	7.94	9.00	9.33
Ethylbenzene	15.05	21.26	5.85	9.80
Styrene	53.73	45.17	67.28	60.02
Xylene	3.20	3.24	2.19	2.49
Propylbenzene	3.34	4.63	1.26	2.18
Methylstyrene	10.82	12.16	8.90	9.87
Trimethylbenzene	0.76	0.54	0.39	0.27
Indane	0.22	0.26	0.18	0.13
Indene	0.10	0.12	0.06	0.08
C10	3.37	2.72	3.25	2.90
C11-C13	1.90	0.83	1.07	1.73

Fig. 1. DTG and TG curves of single PP and PS with HNZ catalyst at a heating rate 10 ℃/min.