Recycling of extruded polystyrene wastes by dissolution and supercritical CO2 technology

#HISFR y,E?:-0-,1,1, 1l¿9 RECYCLING oF POLYSWRENE WASTES BY suPERcRlIcAL Goz TECHNoLoGy ór'' lntern¿tional Symposium on Feedstock Recycting of Potynrerrc Materials 150 Carbon dioxide (99.8%) was purchased from Carburos Melálims España S.A. (MadrkJ, Spain). was out C. Gutiénezt, M.T. Garciar, l, Graciar, A. de Lucasr and J.F. Rodríguezz Solubility determination car¡ied by thsrmogravimet¡ic analysis (TA-DSC Q 100). Glass tubes prepare saturaled solutions ol PS pellets were used lo I oeparlment of Chemical Engineering. 2 lnslilule for Chemical and Environmental Technology [1].University ol Castilla-La with excess solid solute in differenl terpene oils. The Mancha. Facutty of Chemistry. Avda. Camito José Ceta 12, 13071 Ciudad R!á, Spa¡n. e-mail:Crisline.Gutienez@udm.es iiirr. Abstract ii¡,1;r. tiTrr. ri'q rliir- llii i:_: l1,r:r i:1 .!i' r'J Polystyrene (PS) is currenlly used as packaging, insulat¡ng and stodng malerial in var¡ous industríal domeslic fietds, As a resuf, a large guantity ol PS wasles ,b produced gaslic u/asles are nol usuelty biodegraddb,e, so I is nacessary lo suggesf a technology lo recycle lhem. Landñll and incineratbn arc reasonable cnéap meinods, bú not environneniatty acceptable, lherelorc, aftemalive nelhcds fot Nyner recycling arc requircd. Dissolulion and shrink¡ng w¡lh nalural solvenls is e cheap añ effíc¡ent process lor polynnr waste ÍEnagefltenl, pg ü .L:: r tnuq has been sti¡die4 pS h disso'ived beller in lhe wasles show an impoflanl wlume rcduclion ol more than 1N lirnes. As I nonpolar solvents of the terpene family, as p-cymene,liñorcne or aalnttandrene, sluwing solubitity vatues arwnd 0.J g/ml. HoweveL polar lerpene solvenlq wfh hbh lendency lo form hydrogen bonds, as geran,iol'ot t¡natod, present sdub¡lily values ol 0.NS glnl. For lhis reason, nonpdat létpene ols n€le serecfed fo study piS disso/ulion pmcess. Finally, solvanl can be recovercd by means of superctit¡cdt lechndogy, in oder to obiain a potymer iith a raduced volune and wilhout degndatíon ol lhe Nlyneric chains. Kewoñs: L The two main allern¿l¡ves for handling polymer wastes are energy recycling, where wasles are incineraled, and mechanical recycling. Unfortunetely, lhis last method of recovery are oflen more expensive than using vhgin plastics [4, 51. An intorest¡ng and cost+ñective alternative cañ be the d¡ssolution ol lhe foam wastes wilh suilable solvenls, to get a volume reduct¡on without degradation ol polymer chains. Furlhermore if the foam shrinking is carried oul in house of the residue producer, the transporlalion would be more eff¡cienl lhan in conventional recycling methods [61. Aomatic compounds, such as loluene or benzene, are good solvenls of PS foams [7], but they are nol environmentally friendly and would limil the furlher applical¡on of recycled PS. for example, in food packaging. Thus, the use of a 'green' solvent would avoid lhose difliculties. Th6 employment of a nah,ral solvent for the treatment of PS wasles could kansform lhe dissolut¡on of PS wastes in an envionmentally friendly lechnology. By this reason, terpene oils have been reportsd as an anractive alternaüve lor PS loam recycling f8¡. However, solvenl removal process by atmospheric or vacuum d¡st¡llal¡on presents several d¡sadvenlages as the lormation of undeskable by- a at a ,unction of volatilizalion rate of temperafure, the first one belongs to of solvenl and the lasl one to PS demmposition. Solvent removal was caried oul in an experimental seta stainl€ss-sleel high pressure vessel wilh an internal volum€ of 350 ml. A pressure up wltich consists ol indicator having a range from 0 lo 250 bar end accuracy of 10 ber is used for ths pressure measuremenls. The I products as a consequence of thernel degradation of polymer cha¡ns, es well as imporlanl energy consumption and a worsl plastic quelity, due lo certain level of solveñt res¡dues [6, 9]. To avoil all ol these disadvantages, elim¡nation ol the solvent with liquid or supercdtical COz can be considered as an attractive eltsmative process, because ol loa operating temperatures and the facl of leaving no solvents ¡esidues in the recovered PS. Also, the use of COz is very atlractive br the polymer solvonl separalion since is capable of sw€lling the polymar making access¡ble lhe intemal part of the polymer bulk to Results and Discussion The first stage consisls ¡n the dissolul¡on of polystyrene with suitable solvents. A good solvent lor $e recyding of extruded polystyrene should have high dissolution ability and high volat¡lity $al will allow its removal with minimum temperature to avoid chain degradalion. By this reason, a sel ol exp€riments was carried oul lo check which lerpenic solvents could be used to dissolve PS [61. Table 1 shows the values of the solubility of PS in diffarent lerpenic solvents al 25oC. Table 1. Experimental solubility of PS in terpene oils 1121. citrus oil wilh supercrilical COz in order to obtain lerpeneless cilrus oil. The operaling temperalure and pressure were in the range 313K.333K and 8.8.11.8 MPa. At such condilions, limonene was exlracted seleclively, showing a solubility value ol 5 mglg conlirming that COz is a very good solvent for terpenes and exh¡bits relalively high solubility in COz. On lhe olher hand, solubil¡ty of PS in COz has also been studied by several authors [14,15,16,17]. ln alt case a poor solubility of the polymer in COz has been observed [14,161. The general mnclusion would be th¿t PS w¡th very low molecular weight is slightly soluble in Coz, for instanc€ 500 g/mol exh¡bit a ralher small solub¡l¡ty tower than 0.1%, while for higher molecular webhts (>1850 g/mol) PS is completely ¡nsoluble in COz [14,15,,l61. study were anisole, cinnamaldehyde, . p-cymenE, eucalyptol, geraniol, limonene, linalciol, o-phellandrene, o. pinene, y-lerpinena and a-lerpineol: all of lhem were suppl¡ed by Sigma.Alddch and were used without further purifcation. [2] N.Kiran, E.Ekinci, ánd C.E.Snape. Resources, Conservation and Recyd¡ng, Vol. 29, 2000, p. 273. [3] Z.Zhibo, S.Nishio, Y.Morioka, A.Ueno, H.ohkita, Y. Tochihara, T.Mizush¡ma, N. Kakuta. Catalys¡s Today, Vol. 29, l9g$, p.303. [4] J.B€ndrup. Recycling and remvery of plast¡cs. G.Menges, Mun¡ch, 1996, p.393. [5] M.Lee. Recycl¡ng polymer wasle. Chemistry in Britain, Vol. 7, 1995, p. 515. M.T.Garcia, G.Duque, l.Gracia, A.d€Lucas, J.F.Rodriguez. Joumal of Material Cycles and Wasle Managemsnl, Vol. 1 1, 2009, p. 2. [6] f¡l A.Karaduman, E.H.$¡mgek. B.Qigek, A,Y.Bilgesú. Journat p.273. fElT.Noguchi, Y.lnagaki, M.M¡ysshita, H.Walanabe. Packag¡ng Technology and Science, Vol. 1 1, 199E, p. 19. l9l M.T.Garcia, Lcracia, G.Duque, A.de Lucas, J.F. Rodrlguez. Waste Managment, Vol. 29, 2009, p. 1814. ll0lA.D.Shine, Gelb, Jr. M¡croencapsulation process using supercritic€l ffuids. United States Patent 5766637. 1998. of Supercril¡cal Fluids, Vol.23, 2002, p. 1 1. flllJ.D.C.Francis€o, B.Sivik. The Journat [12lE.V.Lassak. J.J.Brophy. Flavou¡ and Fragance Joumal, Vol. 19, 2004, p.12. & 1906. [13lM.Sato, M.Goto, T.Hirose. lndustriat Materlals and Methods in his R.K.Balakrishnan, C.Guria. Polymer Degradet¡on and Slability, Vol. 92, 2007, p. 1583. of Analylical and Applied Py¡olys¡s, Vol. 62, 2A02, most appropriale solvents. Th€ solvenls testod are References the COz [101. Polyslyrsne, pellels with a weight average molecular w€ighlbf 280000 g/mol were suppl¡ed by Sigma.Atdrich. because all of lhem Salo and col. (1995) [13] have sludied Ihe exkaclion ol lll This work proposes a globel process for polystyrene reryding in two st€ps: a polystyrene dissolution wilh suileble solvents lollowed by solvenl €liminaüon by sup€rcriticel fiuids. ln order to develop e treen pocess' the constituents of essenllel oils were selecled as lhe 2. [1ll monoterpene hydrocarbons with low molecular we¡ght, high vapor pressure and low polarity, factors that favor solubility in dense gases, espec¡elly in supercritical Coz 3. lnlroduction has great interesl in order lo prevenl the envhonmenlal pollution and to preserve natural soorces [1, 2, 31. to lower solubil¡ty rales tempsrature is measured using a temperalure indic€lor with an accuracy of t 0.1 K. Polyslyrene, lerpene oil, supercrilical COz. ln the lasl years, disposal of lhe non-biodegradable plaslic wasles has been recognized as a se¡ious environmenlal problem. The recycling of lhese wasles tubes were sealed lo prevent evaporat¡on of solvenls and placed in a thermostat¡c balh al a conslanl temperature; they w€re ellowed to setlle about 48 hours lo ensure equilibrium. For each tube, two samples were wilhdrawn and weighted from the dear ssturated solution to minimize experimental errors. The solvent ratio in the sample was determined by lhermal gravimetric anelysis where samples were heated from lhe room temperalure 6000C heating l0oC/min. Thermogravimetric analysis showed lwo w€ight losses as anelyzed the solubllity of the selected lerpene oils in COt, observing thal they are fully m¡sc¡ble with dense gas, and the solubility increases in lemperafure, lead to As oxpected, tñe solvents wiür polar groups ¡n their struclure do not dissolve lhe PS (geraniol and linalool) or exhibil a lower solubility potential. Linrcnene, y-lerpinene, ptymene and o-p¡nene exhibit similar solubility values being good all€rnatives to carry oul the rerycling process thal is why nonpolar terpene oils were selected lo study PS dissolution process. The second stage br the recycling of PS wastes consists of solvent removal by means of dense C0¡. Thus, it was Eng¡neering Chemical Roseardr, Vol. 35, 1996, p. 1 llal M.L.O'Ne¡ll, O.Cao, M.Fang, K.P.Johnsr¡rn, S.P.Witkinson, C.D.Smith, J.L.Kersdner, S.H.Jureller. lndustrial and Engineering Chemisry Research, Vot. 37, 199S, p. 3067. ll5lC.F.K¡óy, M.A.McHugh. Chemical Reviem, Vot. 99, j999. p. 565. l16l F.R¡ndfe¡sch, T.P.DiNoia, M.A.McHugh Chem¡slry, Vol. 100, 1996, p. tSSBi. Jomat of physicat [1fl D.Xu, C.B.Pa*, R.G.Fenlon. Annuat Conference 2005, p. 106, - Technicat ANTEC, Conferene pmeedings, Vot. l,