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TOLEDO (SPAIN)RECYCLING OF POLYSTYRENE WASTES BY SUPERCRITICAL CO2 TECHNOLOGY
C. Gutiérrez1, M.T. García1, I. Gracia1, A. de Lucas1 and J.F. Rodríguez2
1 Department of Chemical Engineering. 2 Institute for Chemical and Environmental Technology [1].University of Castilla-La

Mancha. Faculty of Chemistry. Avda. Camilo José Cela 12, 13071 Ciudad Real, Spain.
e-mail:Cristina.Gutierrez@uclm.esAbstract
Polystyrene (PS) is currently used as packaging, insulating and storing material in various industrial or domestic fields.
As a result, a large quantity of PS wastes is produced. Plastic wastes are not usually biodegradable, so it is necessary tosuggest a technology to recycle them. Landfill and incineration are reasonable cheap methods, but not environmentally
acceptable, therefore, alternative methods for polymer recycling are required.
Dissolution and shrinking with natural solvents is a cheap and efficient process for polymer waste management, thus, PS
wastes show an important volume reduction of more than 100 times. As it has been studied, PS is dissolved better in thenonpolar solvents of the terpene family, as p-cymene, limonene or D-phellandrene, showing solubility values around 0.3
g/ml. However, polar terpene solvents, with high tendency to form hydrogen bonds, as geraniol or linalool, present
solubility values of 0.005 g/ml. For this reason, nonpolar terpene oils were selected to study PS dissolution process.
Finally, solvent can be recovered by means of supercritical technology, in order to obtain a polymer with a reduced
volume and without degradation of the polymeric chains.

Keywords: Polystyrene, terpene oil, supercritical CO2.

1. Introduction
In the last years, disposal of the non-biodegradable
plastic wastes has been recognized as a serious
environmental problem. The recycling of these wastes
has great interest in order to prevent the environmental
pollution and to preserve natural sources [1, 2, 3].
The two main alternatives for handling polymer wastes
are energy recycling, where wastes are incinerated, andmechanical recycling. Unfortunately, this last method ofrecovery are often more expensive than using virginplastics [4, 5]. An interesting and cost-effective
alternative can be the dissolution of the foam wastes withsuitable solvents, to get a volume reduction without
degradation of polymer chains. Furthermore if the foamshrinking is carried out in house of the residue producer,
the transportation would be more efficient than inconventional recycling methods [6].
Aromatic compounds, such as toluene or benzene, aregood solvents of PS foams [7], but they are notenvironmentally friendly and would limit the further
application of recycled PS, for example, in foodpackaging. Thus, the use of a “green” solvent would
avoid those difficulties. The employment of a naturalsolvent for the treatment of PS wastes could transformthe dissolution of PS wastes in an environmentallyfriendly technology. By this reason, terpene oils havebeen reported as an attractive alternative for PS foam
recycling [8]. However, solvent removal process byatmospheric or vacuum distillation presents several
disadvantages as the formation of undesirable by-interiores libro.indd 149

products as a consequence of thermal degradation ofpolymer chains, as well as important energy consumption
and a worst plastic quality, due to certain level of solvent
residues [6, 9]. To avoid all of these disadvantages,
elimination of the solvent with liquid or supercritical CO 2
can be considered as an attractive alternative process,because of low operating temperatures and the fact ofleaving no solvents residues in the recovered PS. Also,
the use of CO2 is very attractive for the polymer solvent
separation since is capable of swelling the polymer
making accessible the internal part of the polymer bulk to
the CO2 [10].
This work proposes a global process for polystyrene
recycling in two steps: a polystyrene dissolution withsuitable solvents followed by solvent elimination by
supercritical fluids. In order to develop a “green process”
the constituents of essential oils were selected as themost appropriate solvents.

2. Materials and Methods
Polystyrene pellets with a weight average molecularweight of 280000 g/mol were supplied by Sigma-Aldrich.
The solvents tested in this study were anisole,cinnamaldehyde, p-cymene, eucalyptol, geraniol,limonene,linalool,
D-phellandrene,Dpinene,Jterpinene and D-terpineol; all of them weresupplied by Sigma-Aldrich and were used without further
purification.

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6th International Symposium on Feedstock Recycling of Polymeric Materials

Carbon dioxide (99.8%) was purchased from Carburos
Metálicos España S.A. (Madrid, Spain).
Solubility determination was carried out by
thermogravimetric analysis (TA-DSC Q 100). Glass tubes
were used to prepare saturated solutions of PS pellets
with excess solid solute in different terpene oils. The
tubes were sealed to prevent evaporation of solvents andplaced in a thermostatic bath at a constant temperature;they were allowed to settle about 48 hours to ensure
equilibrium. For each tube, two samples were withdrawn
and weighted from the clear saturated solution tominimize experimental errors. The solvent ratio in thesample was determined by thermal gravimetric analysis
where samples were heated from the room temperature
to 600ºC at a heating rate of 10ºC/min.
Thermogravimetric analysis showed two weight losses asa function of temperature, the first one belongs tovolatilization of solvent and the last one to PS
decomposition.
Solvent removal was carried out in an experimental setup which consists of a stainless-steel high pressurevessel with an internal volume of 350 ml. A pressure
indicator having a range from 0 to 250 bar and accuracyof ± 10 bar is used for the pressure measurements. The
temperature is measured using a temperature indicatorwith an accuracy of ± 0.1 K.

analyzed the solubility of the selected terpene oils in
CO2, observing that they are fully miscible with densegas, and the solubility increases in temperature, lead tolower solubility rates [11] because all of them aremonoterpene hydrocarbons with low molecular weight,
high vapor pressure and low polarity, factors that favorsolubility in dense gases, especially in supercritical CO 2
[12].
Sato and col. (1995) [13] have studied the extraction ofcitrus oil with supercritical CO2 in order to obtainterpeneless citrus oil. The operating temperature andpressure were in the range 313K-333K and 8.8-11.8
MPa. At such conditions, limonene was extracted
selectively, showing a solubility value of 5 mg/g
confirming that CO2 is a very good solvent for terpenes
and exhibits relatively high solubility in CO 2.
On the other hand, solubility of PS in CO2 has also beenstudied by several authors [14,15,16,17]. In all case apoor solubility of the polymer in CO2 has been observed[14,16]. The general conclusion would be that PS withvery low molecular weight is slightly soluble in CO2, forinstance 500 g/mol exhibit a rather small solubility lower
than 0.1%, while for higher molecular weights (>1850g/mol) PS is completely insoluble in CO2 [14,15,16].

3. Results and Discussion[1] R.K.Balakrishnan, C.Guria. Polymer Degradation andStability, Vol. 92, 2007, p. 1583.
[2] N.Kiran, E.Ekinci, and C.E.Snape. Resources,Conservation and Recycling, Vol. 29, 2000, p. 273.
[3] Z.Zhibo, S.Nishio, Y.Morioka, A.Ueno, H.Ohkita, Y.
Tochihara, T.Mizushima, N. Kakuta. Catalysis Today, Vol.
29, 1996, p. 303.
[4] J.Brandrup. Recycling and recovery of plastics. G.Menges,Munich, 1996, p. 393.
[5] M.Lee. Recycling polymer waste. Chemistry in Britain, Vol.
7, 1995, p. 515.
[6] M.T.García, G.Duque, I.Gracia, A.deLucas, J.F.Rodríguez.
Journal of Material Cycles and Waste Management, Vol. 11,2009, p. 2.
[7] A.Karaduman, E.H.Şimşek, B.Çiçek, A.Y.Bilgesü. Journalof Analytical and Applied Pyrolysis, Vol. 62, 2002, p. 273.
[8] T.Noguchi,Y.Inagaki,M.Miyashita,H.Watanabe.
Packaging Technology and Science, Vol. 11, 1998, p. 19.
[9] M.T.García, I.Gracia, G.Duque, A.de Lucas, J.F.
Rodríguez. Waste Management, Vol. 29, 2009, p. 1814.
[10] A.D.Shine, Gelb, Jr. Microencapsulation process using
supercritical fluids. United States Patent 5766637, 1998.
[11] J.D.C.Francisco, B.Sivik. The Journal of SupercriticalFluids, Vol. 23, 2002, p. 11.
[12] E.V.Lassak, J.J.Brophy. Flavour and Fragance Journal,
Vol. 19, 2004, p.12.
[13] M.Sato, M.Goto, T.Hirose. Industrial & Engineering
Chemical Research, Vol. 35, 1996, p. 1906.
[14] M.L.O'Neill, Q.Cao, M.Fang, K.P.Johnston, S.P.Wilkinson,C.D.Smith, J.L.Kerschner, S.H.Jureller. Industrial andEngineering Chemistry Research, Vol. 37, 1998, p. 3067.
[15] C.F.Kirby, M.A.McHugh. Chemical Reviews, Vol. 99, 1999,p. 565.
[16] F.Rindfleisch, T.P.DiNoia, M.A.McHugh Journal of Physical
Chemistry, Vol. 100, 1996, p. 15581.
[17] D.Xu, C.B.Park, R.G.Fenton. Annual TechnicalConference - ANTEC, Conference Proceedings, Vol. 1,2005, p. 106.

The first stage consists in the dissolution of polystyrene
with suitable solvents. A good solvent for the recycling ofextruded polystyrene should have high dissolution ability
and high volatility that will allow its removal with minimum
temperature to avoid chain degradation. By this reason, aset of experiments was carried out to check which
terpenic solvents could be used to dissolve PS [6]. Table
1 shows the values of the solubility of PS in different
terpenic solvents at 25ºC.
Table 1. Experimental solubility of PS in terpene oils
Solvent
AnisoleCinammaldehyde
p-Cymene
Eucalyptol
Geraniol
Limonene
Linalool-Phellandrene-Pinene-Terpinene-Terpineol

Solubility (g/ml)
0.25780.19570.35290.29770.00290.24730.00590.28790.2901
0.24270.0063As expected, the solvents with polar groups in theirstructure do not dissolve the PS (geraniol and linalool) orexhibit a lower solubility potential. Limonene, J-terpinene,
p-cymene and D-pinene exhibit similar solubility values
being good alternatives to carry out the recycling processthat is why nonpolar terpene oils were selected to study
PS dissolution process.
The second stage for the recycling of PS wastes consistsof solvent removal by means of dense CO2. Thus, it wasinteriores libro.indd 150

References

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