CHEMICAL RECYCLING OF POLYSTYRENE D.S. Achilias, I. Kanellopoulou, P. Megalokonomos Laboratory of Organic Chemical Technology, Department of Chemistry Aristotle University of Thessaloniki, 541 24 Thessaloniki – GREECE E-mail:
[email protected] and
A. Lappas, Ε. Antonakou Laboratory of Environmental Fuels and Hydrocarbons Chemical Process Engineering Research Institute, 570 01 Thermi, Thessaloniki E-mail:
[email protected]
ABSTRACT The recycling of polystyrene is examined using the dissolution/reprecipitation method, as well as pyrolysis. In the first technique, different solvents/non-solvents were examined at different weight percent amounts and temperatures. The recovery of polymer in every case was greater than 90%. Pyrolysis was carried out either conventional (non-catalytic) or catalytic in a laboratory fixed bed reactor, using a model polymer as raw material or commercial products (food retail outlets). The liquid fraction constituted mainly of the styrene monomer, together with a number of secondary materials (aromatic compounds). The potential use of this product as a raw material for the reproduction of polystyrene by polymerization was investigated using DSC and the results compared with that from virgin monomer.
1. INTRODUCTION Recycling of waste polymers has been a topic of interest in the fields of environmental science and technology for some time. The produced amounts of plastic solid wastes in Greece, as well as in most countries, continue to increase despite some increasing attempts to reduce, reuse, recycle and recover. This is mainly due to their wide application in the manufacture of packaging for the food industry as well as in other goods of daily life, since they do not have any side effects on the human organism. However, due to the nature of much contaminated plastic waste it can only be partly recycled into new products. The presently most used way of handling these waste streams is to incinerate them with energy recovery or to use them for land-filling. In the last decade, many environmental regulations have been implemented for a more sustainable recycling oriented society. The approaches that have been proposed for recycling of waste polymers include [1,2]: Primary recycling referring to the “in-plant” recycle of the scrap material of controlled history. Mechanical Recycling, where the polymer is separated from its associated contaminants and it is reprocessed by melt extrusion. Chemical recycling leading in total depolymerization to the monomers, or partial degradation to other secondary valuable materials. Energy recovery as an effective way to reduce the volume of organic materials by incineration. Although polymers are actually high-yielding energy sources, this method has been widely accused as ecologically unacceptable owing to the health risk from air born toxic substances e.g. dioxins (in the case of chlorine containing polymers). The objective of a plastic management policy, in accordance with the principles of sustainable development, should be not only the reuse of polymeric materials but also the production of raw materials (monomers), from which they could be reproduced, or other secondary valuable products. In this sense, among the techniques proposed for recycling of waste polymers the most challenging method is chemical or feedstock recycling and various technologies have been successfully demonstrated and continue to be developed. The recycling of plastic waste by such higher technological routes has the potential to recover higher value end products like the hydrocarbons of the polymer in the form of gas or oil. Polystyrene (PSt) is a major type of thermoplastic used throughout the world in such applications as electrical appliances, thermal insulation, tape cassettes, cups and plates. In Western Europe alone approximately 3.14 million tones of polystyrene are consumed each year (data of 2003) [3]. Polystyrene comprises 12.3% of the plastic content of municipal solid wastes. In addition polystyrene represents a plastic, which may be collected as a single source, for example from food retail outlets and from commercial and industrial premises. In contrast with condensation polymers (i.e. poly(ethylene terephthalate) (PET)), PSt can not be easily recycled to monomer by simple chemical methods. Instead, thermochemical recycling techniques like pyrolysis are usually applied. Thus several authors in literature have reported either thermal or catalytic cracking of PSt to obtain the monomer styrene and a gasoline fraction [4-7]. Either acid-type [8] or solid bases [9] were used. In the present investigation the chemical recycling of polystyrene was examined using two different methods: the traditional method of dissolution/reprecipitation and the more challenging technique of pyrolysis. The first belongs to the mechanical recycling techniques while the second to chemical/feedstock recycling. During the first technique the polymer can be separated and recycled using a solvent/non-solvent system. For this purpose different systems were examined at different weight percent amounts and temperatures. Furthermore, conventional (non-catalytic) and catalytic pyrolysis was carried out using either model polymer as raw material or commercial products (mainly food retail outlets). The experiments were carried out in a laboratory fixed bed reactor. The potential use of the oil produced as a raw material for the reproduction of PSt by polymerization was investigated using DSC and the results compared with that from virgin monomer. Conclusions are very encouraging concerning alternative techniques of waste polystyrene recycling.
2. MATERIALS AND METHODS 2.1 Materials Model polystyrene obtained from Aldrich (number average molecular weight 140 000) and different commercial products made from polystyrene (PSt) or expanded polystyrene (EPS). The solvents used (toluene, xylene, n-hexane, methanol) were of reagent grade. The catalysts used were BaO (Aldrich) with purity 97% and a commercial FCC catalyst (total surface area 178.4m2/g, zeolite area 58.5 m2/g). 2.2 Dissolution/reprecipitation technique In a first approach, model polystyrene was used together with different commercial products containing polystyrene. Xylene and toluene were used as solvents, while methanol and n-hexane as non-solvent. Some other parameters include concentration of the polymers: 20% w/v, solvent/nonsolvent volume ratio: 1/3 and different dissolution temperature below the boiling point for each solvent (140oC for xylene and 110oC for toluene). The experimental process comprised: the polymer (4 gr) and the solvent (20 mL) were added into a flask equipped with a vertical condenser and a magnetic stirrer. The system was heated for 30 min to the desired temperature. Then, the flask was cooled and the solution of the polymer was properly poured into the non-solvent. The polymer was re-precipitated, washed, filtrated and dried in an oven at 80oC for 10 h. The recycled polymer was obtained in the form of powder or grains. 2.3 Pyrolysis All experiments took place in the Laboratory of Environmental Fuels and Hydrocarbons, situated in CPERI, Thessaloniki, Greece. The reactor (Fig. 1) was filled with 0.7 g catalyst, or glassbeads for the non-catalytic tests and the piston was filled with polymer (1.5 g). Glasswool was placed in the bottom of the reactor, the top of the piston and inside the bed in order to separate the catalyst and the polymer bed. The reactor and the piston were connected and placed into the oven. The system was always heated in the presence of N2 (30 mL/min) and, by using a temperature controller the temperature of each zone of the furnace was controlled. As soon as the reaction temperatures were achieved, polymer entered the reactor and the experiment started. The time of the experiment was 17 min and the reaction temperature was 510oC. At the end of the experiment, purging (30 min) was performed. Both the experiment (100 mL/min) and purging (30 mL/min) were performed in the presence of N2. The pressure was controlled, before and after the entrance of the piston into the reactor to identify potential blockage. The liquid products were collected in a liquid bath (-17 °C) and quantitatively measured in a preweighted glass receiver. The gaseous products were collected and measured by water displacement. The amount of coke formed was measured by direct weighting. The reaction temperature was kept at 510 °C. The liquid samples were analysed by GC/MS analysis in a HP 5989 MS ENGINE (Electron energy 70eV; Emission 300V; Helium flow rate: 0.7cc/min; Column: HP-5MS(30m x 0.25mmID x 0.25 μm)) for the identification of compounds in the organic phase. Using the GC/MS technique the qualitative analysis of the organic fraction was performed. Gaseous products were analysed in a HP 6890 GC, equipped with four columns (Precolumn:OV-101; Columns: Porapak N, Molecular Sieve 5A and Rt-Qplot (30m x 0.53mm ID) and two detectors (TCD and FID). The chromatograph was standardized with gases at known concentrations as standard mixtures.
PYROLYSIS UNIT
Figure 1. The fixed bed reactor system 2.4 Re-polymerization of the pyrolysis oil Azo-bisisobutyronitrile (AIBN) was used as an initiator of the free radical polymerization of styrene. Solutions of either pure styrene (monomer), or the liquid fraction from pyrolysis with initiator were prepared at a concentration of AIBN 0.1 M. Polymerization was investigated using the DSC, Pyris 1 (from Perkin-Elmer) equipped with the Pyris software for windows. Indium was used for the enthalpy and temperature calibration of the instrument. Polymerizations were carried out at 90oC. The reaction temperature was recorded and maintained constant (within 0.01 oC) during the whole conversion range. The samples were weighted (approximately 10 mg) sealed and placed into the appropriate position of the instrument. The reaction exotherm (in normalized values, W/g) at a constant temperature was recorded as a function of time. The rate of heat release (d(ΔΗ)/dt) measured by the DSC was directly converted into the overall reaction rate (dx/dt) using the following formula: 1 d (H ) dx dt H T dt where ΔHT denotes the total reaction enthalpy (690 J/g for styrene). The polymerization enthalpy was calculated by integrating the area between the DSC thermograms and the baseline established by extrapolation from the trace produced after complete polymerization (no change in the heat produced during the reaction). 3. RESULTS AND DISCUSSION 3.1 Recycling of polystyrene by the dissolution/reprecipitation technique Two solvent/nonsolvent systems and three dissolution temperatures were investigated. The effect of temperature as well as solvent/nonsolvent used on the % recovery of polystyrene from a model polymer and several commercial products appears in Table 1. The polymer concentration was always 20 %.-wt. It is obvious that at all different experimental conditions and for all commercial
samples examined the polymer recovery was always high. An increase in dissolution temperature leads to increased polymer recovery values. TABLE 1. Polystyrene recovery by the dissolution/reprecipitation technique Model PSt Plastic Plastic Packaging Plastic bowl Solvent/Nonsolvent Temp. (oC) Glass container for foods of yoghurt (EPS) (PSt) (EPS) (PSt) Toluene/n-Hexane Toluene/n-Hexane Toluene/n-Hexane Xylene/methanol Xylene/methanol Xylene/methanol
20 50 100 20 50 100
84.0 87.7 92.5 89.2 92.8 97.9
93.6 95.1 96.0 96.4 99.6 99.8
76.2 90.6 95.7 95.5 96.9 99.7
89.1 93.1 95.7 90.1 95.4 96.2
96.4 97.0 96.5 94.3 97.7 96.8
Comparing the FT-IR spectra of the solid obtained from commercial samples with that of model polystyrene identified the polymer recovered with this technique. The interpretation of spectra showed that in all cases the product of recycling was polystyrene. Indicative spectra of a plastic glass and model PSt are presented in Figure 2. 100
% Transmittance
80
60
40
20
Model PSt Commercial product (plastic glass from PSt)
0 0
500
1000
1500
2000
2500
3000
3500
4000
-1
Wavenumber, cm
Figure 2. FT-IR spectra of model polystyrene and the polymer recovered from a commercial plastic glass 3.2 Recycling of polystyrene by pyrolysis Thermal cracking or pyrolysis, involves the degradation of the polymeric materials by heating in the absence of oxygen (usually in a nitrogen atmosphere). During pyrolysis at increased temperatures, depending on polymer type, either end-chain, or random scission of the macromolecules occurs. In the first case (occurring in poly(methyl methacrylate)) the monomer can be produced in a large amount, while in the second (polyethylene) the amount of monomer produced is very low. Polystyrene degradation is in the middle, meaning that both end-chain and random scission occur
simultaneously. Thus, the amount of monomer produced depends upon the experimental conditions and range between 40-70%. In this investigation results are presented for the thermal and catalytic pyrolysis of model polystyrene using an acid FCC catalyst and a base-type BaO. In addition, pyrolysis of several commercial samples was carried out and all results on the product yield appear in Table 2. TABLE 2. Product yield from the thermal and catalytic pyrolysis of model polystyrene and commercial products containing polystyrene. Pyrolytic process/ sample
Temperature (οC)
Gas (wt.%)
Liquid (wt.%)
Coke (wt.%)
Residue (wt.%)
Model polystyrene Thermal
510
2.5
89.8
0.1
7.6
Catalytic, FCC
510
2.0
83.7
9.8
4.5
Catalytic, BaO
510
0.7
87.0
0.3
12.0
Commercial products Plastic glass (EPS)
510
6.0
86.7
0.9
6.4
Plastic container (PSt)
510
3.6
49.9
30.8
15.7
From Table 2, it is seen that the oil produced is much larger compared to gas and is in the range 84-90% in all but the plastic container samples. Commercial products leave more residue and coke. The composition of the gaseous fraction is presented in Table 3. TABLE 3. Composition of the gaseous fraction of the thermal and catalytic pyrolysis of model polystyrene and commercial products containing polystyrene (%) Catalytic FCC
Catalytic BaO
Plastic glass (EPS)
Plastic container (PSt)
0.0 0.0 0.0 21.8 0.0 57.9 0.0 11.9 0.0 8.4 0.0 0.0
26.9 0.0 0.0 23.0 0.0 29.5 0.0 18.6 0.0 0.0 2.0 0.0
81.6 0.0 0.0 4.8 0.0 6.2 0.0 2.5 2.7 1.2 0.0 1.0
0.0 0.0 52.0 7.2 0.0 0.0 0.0 2.6 0.0 2.8 28.9 6.5
0.0 55.4 0.0 6.2 2.0 10.0 0.0 4.7 14.6 1.1 3.1 2.9
100.0
100.0
100.0
100.0
100.0
Thermal
Hydrogen, H2 Carbon dioxide, CO2 Carbon monoxide, CO Methane, CH4 Ethane, C2H6 Ethylene, C2H4 Propane, C3H8 Propylene, C3H6 Butane-Butylene, nC4 Pentane-Pentene, nC5 Isopentane-Isopentene, iC5 C6 Total
It is observed that cracking of model polystyrene does not lead to the production of CO and CO2, since in this polymer molecule there are not any oxygen atoms. However, in the commercial samples either CO, or CO2 have been detected meaning the appearance of additives containing oxygen. Furthermore, in the plastic glass which constitutes of expandable polystyrene it was evident the existence of pentanes in a rather large amount (32%), verifying thus the introduction of mainly isobutene as an expanding agent [1]. The gases produced from cracking of the model PSt are mainly H2, CH4, ethylene and propylene (>90%). Hydrogen is only produced from catalytic cracking. Moreover, the compounds identified in the liquid fraction from pyrolysis appear in Table 4. It was observed that the largest amount of monomer was obtained during thermal pyrolysis of model polymer and EPS as well as with the BaO catalyst. In the commercial samples the % amount of monomer in the plastic glass was very high while much less in the other plastic container. Several other aromatic compounds were identified differing mainly by the existence or not of catalyst. TABLE 4. Aromatic compounds identified in the liquid fraction of the thermal and catalytic pyrolysis of model polystyrene and commercial products containing polystyrene (%) Compounds
Styrene (monomer) 2,4-Diphenyl-1-butene (dimer) 2,4,6-Triphenyl-1-hexene (trimer) Benzene Toluene Ethylbenzene a-Methylstyrene Xylene Indane, Ιndene, etc. 1,2-Diphenylethane 1-methyl-1,2 Diphenylethane 1,3-Diphenylpropane 1,1’-Diphenylpropene Cumene Naphthalene and derivatives Phenanthrene and derivatives Other aromatic compounds
Thermal
Catalytic FCC
63.9 14.0 2.2 2.0 0.5 2.1 0.3 2.2 1.1 0.6 0.2 10.9
45.1 1.9 0.3 1.9 3.1 6.3 16.8 4.2 0.9 0.5 0.5 1.1 1.7 0.8 2.6 12.3
Catalytic Plastic glass Plastic container BaO (EPS) (PSt) 69.6 30.4 -
70.0 9.0 5.2 0.1 2.5 1.5 2.3 0.2 0.8 0.4 0.8 0.7 0.3 0.3 6.1
53.3 13.2 3.5 5.6 1.9 5.9 2.5 2.1 1.4 2.8 0.2 0.4 7.2
3.3 Re-polymerization of the oil fraction Finally, some results are presented on the polymerization of the oil fraction received from pyrolysis and they are compared to the polymerization of fresh styrene. Experimental data of the reaction rate versus time appear in Figure 3. It can be seen that the product of thermal cracking with the higher amount of styrene monomer results in a profile resembling that of the pure styrene, though not exactly the same. The total heat released during polymerization was 690, 564, 357 and 97 J/g for the pure styrene, product of thermal, catalytic with BaO and FCC pyrolysis, respectively. These results
correspond to final double bond conversion equal to 100, 82, 52 and 14%, respectively. Therefore, the possibility of using directly the oil product from pyrolysis for the reproduction of polystyrene is under question.
4
Reaction Rate (x10 ), s
-1
6 pure Styrene liquid product of thermal pyrolysis liquid product of catalytic pyrolysis (BaO) liquid product of catalytic pyrolysis (FCC)
5 4 3 2 1 0 0
20
40
60
80
100
120
140
Polymerization time, min
Figure 3. Polymerization rate versus time of different oil fractions obtained from pyrolysis of model polystyrene compared to polymerization of pure styrene monomer. Initiator: AIBN 0.1M, reaction temperature 90oC. 4. CONCLUSIONS The chemical recycling of polystyrene was examined by both a dissolution/reprecipitation technique and pyrolysis. The first leads to high recovery of polymer with the disadvantage of using large amounts of organic solvents. Pyrolysis seems to be the most promising technique resulting in an oil fraction with a great percentage of styrene monomer. The possibility of using directly this product for the reproduction of PSt is not so clear. Aknowledgements This work was funded by the Ε.Κ.Τ. / Ε.Π.Ε.Α.Ε.Κ. ΙΙ in the framework of the research program PYTHAGORAS II, Metro 2.6. REFERENCES [1] [2] [3] [4] [5] [6] [7] [8] [9]
Scheirs J. (1998) “Polymer Recycling”, J. Wiley & Sons, W. Sussex. Achilias D.S. and G.P. Karayannidis (2004) Water, Air & Soil Pollution: Focus, 4, 385. http://www.plasticseurope.org Kaminsky W., M. Predel and A. Sadiki (2004) Polym. Degrad. Stab., 85, 1045. Serrano D.P., J. Aguado and J.M. Escola (2000) Appl. Catal. B: Environm., 25, 181. Audisio G., F. Bertini, P.L. Beltrame and P. Carniti (1990) Polym. Degrad. Stab., 29, 191. Liu Y., J. Qian and J. Wang (2000) Fuel Proc. Techn., 63, 45. Williams P.T. and R. Bagri (2004) Int. J. Energy Res., 28, 31. Zhang Z., T. Hirose, S. Nishio, Y. Morioka, N. Azuma, A. Ueno, H. Okhita and M. Okada (1995) Ind. Eng. Chem. Res., 34, 4514.
