High-pressure rheology of polymer melts containing supercritical carbon dioxide

Transcription

1 Korea-Australa Rheology Journal Vol. 18, No. 2, June 2006 pp Hgh-pressure rheology of polymer melts contanng supercrtcal carbon doxde Sang Myung Lee, Jae Ho Han, Kyung Yl Km, Young Joon Ahn and Jae Wook Lee* Appled Rheology Center, Dept. of Chemcal Engneerng, Sogang Unversty, 1 Shnsu-Dong, Mapo-Gu, Seoul , Korea (Receved May 6, 2006) Abstract Supercrtcal carbon doxde (scco 2 ) has advantages of beng ncorporated n polymer wth hgh solublty and of beng recovered easly by depressurzng. ScCO 2 reduces the vscosty of polymer melt and t s expected to be use as a plastczng agent. In ths work, we studed on the effect of scco 2 on the rheologcal propertes of polymer melts durng extruson process. Slt de attached to twn screw extruder was used to measure the vscosty of polymer melts plastczed by supercrtcal CO 2. A gas njecton system was devsed to accurately meter the supercrtcal CO 2 nto the extruder barrel. Measurements of pressure drop wthn the de, confrmed the presence of a one phase mxture and a fully developed flow durng the measurements. The vscosty measurement of polypropylene was performed at expermental condtons of varous temperatures, pressures and CO 2 concentratons. We observed that melt vscosty of polymer was dramatcally reduced by CO 2 addton. Keywards : rheology, supercrtcal flud, extruder, solublty, polypropylene 1. Introducton Supercrtcal flud s defned as a flud that s above ts crtcal temperature and pressure. Supercrtcal fluds have characterstcs such as gas-lke dffusvty and vscosty and lqud-lke densty. In partcular, supercrtcal CO 2 has a wde range of applcatons due to envronmentally frendlness, lower crtcal pont, relatvely low cost, non-toxcty and non-flammablty compared to other supercrtcal fluds. Vscosty reducton of polymers has been of great nterest n the realm of polymer processng because hgh vscosty s a major obstacle for the processng of hgh molecular weght polymers or complex mxtures of partcle flled polymers or pastes. Usually, ths problem s resolved by ncreasng the temperature or by the addton of plastczng agents. However, ncreasng the temperature requres hgher energy consumpton and may lead to thermal degradaton. Added plastczers usually reman n the product, thus affectng ts propertes and performance. Supercrtcal CO 2 can be used as plastczng agent that lowers the vscosty of varous polymer melts (Chou et al., 1985; Lee et al., 1999; Lee et al., 2000). The method of removal of carbon doxde used as a plastczer from the polymerc product s smple, economc and safe compared to the conventonal plastczer removal methods such as costly dryng and hazardous solvent removal processes, *Correspondng author: jwlee@sogang.ac.kr 2006 by The Korean Socety of Rheology whch s very mportant n the processng of polymer-based materals. Upon release of pressure from varous processes the CO 2 wll dsspate as a gas and only the fnal product wll reman. Recently, supercrtcal CO 2 has been appled to the plastcs ndustry such as synthess, foamng and blendng due to ts varous advantages. To apply supercrtcal CO 2 n polymer processng applcatons, t s essental to know the physcal propertes of polymer/co 2 system. Ths study s focused on the measurement of rheologcal propertes of CO 2 -plastczed polymer melts. 2. Theory 2.1. Method of vscosty calculaton In capllary de flow, the melt vscosty can be determned from the measured pressure drops, P and volumetrc flow rate of the polymer n a slt de, Q, usng the followng equatons; H τ w = dp (2) 21 ( + H W) dx where x s the length of the de, H s the slt heght and W s the slt de wdth. γ w s the wall shear rate, whch s converted from the apparent shear rate,, by the Rabnowη = τ w γ w τ w denotes the shear stress at the slt de wall whch s defned by γ ap (1) Korea-Australa Rheology Journal June 2006 Vol. 18, No. 2 83

2 Sang Myung Lee, Jae Ho Han, Kyung Yl Km, Young Joon Ahn and Jae Wook Lee tsch correcton. γ ap γ w d lnγ ap = 3 d lnτ w The apparent shear rate s gven by γ ap = 6Q WH 2 Q denotes the volumetrc flow rates of the neat molten polymer or polymer/co 2 solutons. Dvdng the mass flow rate of the polymer by ts densty gves the value of Q. It s mportant to note that the densty of polymer/co 2 soluton s a functon of temperature, pressure and concentraton of dssolved CO 2. An approprated correcton of the volumetrc flow rates s needed to calculate accurate vscostes n consderaton of temperature, pressure and CO 2 dssoluton effects. The equaton of state was used to calculate the denstes of both neat polymer and polymer/ CO 2 solutons. In ths study, the Sanchez-Lacombe equaton of state (S-L EOS) was employed (Sanchez et al., 1978; Lacombe et al., 1976). S-L EOS s used wth the mxng rules and t wll be ntroduced to Secton 2.2. The equaton of state for polymer alone s derved by fttng S- L EOS to a PVT data of the neat polymer. To calculate the characterstc parameters and derve the equaton of state for polymer/co 2 solutons, the bnary nteracton parameter, k 12, between the polymer and CO 2 s requred. The nteracton parameter was obtaned through data of CO 2 - solublty nto the polymer Sanchez-Lacombe Equaton of State (S-L EOS) The Sanchez-Lacombe equaton of state s gven by: ρ 2 + P + T ln( 1 ρ ) r ρ = 0 where ρ, P and T are reduced densty, pressure and temperature, respectvely. The reduced varables are defned n terms of characterstc parameters: T = ---- T T * = ε * R T * P = ---- P P * = ε * v * P * ρ ρ = ---- ρ * = M rv * ρ * ( ) where T *, P * and ρ * are characterstc temperature, pressure and densty, respectvely. The parameter ε * s the nteracton energy, v * s the characterstc volume of a lattce ste and r s the number of lattce stes occuped by a molecule of molecular weght M. In the Sanchez-Lacombe theory, T *, P * and ρ * are derved from ε *, v * and r, as shown n Eq. 6. In applcaton of the S-L EOS, however, T *, P * and ρ * are (3) (4) (5) (6) often determned drectly as the pure component parameters by an optmzaton procedure. The EOS for the mxture s formally dentcal to that of a pure component. However, mxng rules are employed for calculatng the characterstc parameters and r of the mxture. In formulatng the mxng rules, two materal balance equatons for the close-packed molecular volume of each component and can be expressed as r v * * = r o v. Ths o relatonshp guarantees that an molecule occupyng r * stes of volume v n ts pure state wll occupy the same volume, r stes of volume v * n the mxture. The second rule keeps the total number of par nteractons n the closepacked mxture the same as the sum of the par nteractons of the components n ther pure close-packed states,.e., r o N = rn where N s the number of molecules and N s the total number of molecules n the lattce. The combned result of these materal balances s to ntroduce a surface area effect so that a component n a bnary mxture wth a larger mer volume can have more nteractons n the mxture than n ts pure state. The average close-packed mer volume n the mxture s defned by: v * * = φ o v o where φ = r o N ( r o N ) s the close-packed volume fracton of component n the mxture. The average number of lattce stes occuped by the mxture, r, s weghted by mole fractons: r = x r The characterstc pressure P * of the mxture s taken as parwse addtve: P * * * * = φ φ j P j ; P j = ( P * P j ) 1 2 ( 1 δ j ) j where φ s the volume fracton of component n the mxture, gven by φ = r N ( r N ). The bnary nteracton parameter δ j, the only mxture parameter, corrects the devaton of the mxture s P j * from the geometrc mean of components characterstc pressures, P * * and P j. In the applcaton of the EOS to a bnary or pseudobnary mxture, v *, r and P * for the mxture are calculated from Eqs. 7~9 and then the other three parameters ε *, T * and ρ * are determned from the relatonshps gven n Eq. 6. The phase equlbrum s defned by equatng the chemcal potentals of gas n the two phases: µ 1G ( TP, ) = µ 1P ( TPφ,, ) where the superscrpts G and P represent the gas and polymer phases, respectvely; the subscrpts 1 represents the lghter component (gas). The upper phase s assumed to be pure. The dfference of the chemcal potentals of component 1 n the two phases, whch must vansh at equlbrum, s gven by: (7) (8) (9) 84 Korea-Australa Rheology Journal

3 Hgh-pressure rheology of polymer melts contanng supercrtcal carbon doxde µ lnφ RT 1 1 r = = + φ 2 + r 1o ρ X 1 φ 2 + ṽρ lnρ ṽ 1ρ 1lnρ 1 + (10) * * * where X 1 = ( P 1 + P 2 + 2P 12 )v 1* RT s the only parameter contanng δ j. 3. Expermental r 2 ρ ρ o 1 r P 1( ṽ ṽ 1 ) ṽ( 1 ρ )ln( 1 ρ ) ṽ 1 ( 1 ρ 1 )ln 1 ρ 1 T 1 T 1 ( ) 3.1. Materals The polymer used n ths study was polypropylene (HP602N, Polymrae) whose densty was 0.9 g/cm 3 and MFR was 12.5 g/10 mn. The average molecular weght, M w, was 222,400 and the polydspersty rato, Mw Mn = Carbon doxde (99.99% purty) was used as a supercrtcal flud n the experments Expermental procedure The expermental apparatus used s based on the extruson rheometer developed frst by Han and Ma (1983), and later used by Lee et al. (1999) and Park et al. (1998) and Royer et al. (2000). The vscostes of the neat PP and PP/ CO 2 solutons were measured usng an extruson rheometer equpped n co-rotatng twn screw extruder. The ntermeshng co-rotatng twn screw extruder (model TEK 25 from SM Platek Co.) was specally desgned for the polymerc compoundng, where CO 2 was njected nto a meterng zone of ts barrel and pressure transducer was attached to barrel as shown n Fg. 1. The barrel pressure at the njecton pont s montored for a stable njecton of CO 2. The extruder has 10 barrel sectons and the temperature of each secton s controlled separately. Its screw dameter s 25 mm and L/D s 41. To nject the CO 2 nto the extruder under precsely controlled condton, the njecton system s requred. The njecton system conssts of a CO 2 gas cylnder connected n seres to a syrnge pump (model 260D from ISCO) and a back pressure regulator. Wth the pump supplyng CO 2 set to run at a constant flow rate, the back pressure regulator acts mantanng constant pressure. When the system reaches steady state, the njecton of CO 2 occurs at both constant pressure and constant flow rate. In order to nject supercrtcal CO 2 nto the barrel, pressure transducer port on the barrel was used and a specal njecton devce was desgned. The preparaton of polymer/co 2 solutons can be acheved only at pressure above the solublty pressure. Even though the operatng pressure s mantaned over the solublty lmt, the actual soluble amount of CO 2 mght be less due to the lmted resdence tme durng the extruson. The prepared solutons must be mantaned over the solublty pressure through the extruson process n order to prevent the precptaton of CO 2. Mostly, the conventonal operatng pressure of twn-screw extruders, however, s relatvely low when t s compared to that of sngle-screw extruders. Therefore, specal screw confguratons are requred not only for the pressure generaton but also the stable njecton of CO 2. Fg. 2 shows a schematc of screw confguraton used n ths work. Ths confguraton was desgned for preparng polymer/co 2 and polymer blend/ CO 2 solutons n a twn-screw extruder. It should be noted that a slt de and a nozzle are always attached to ths confguraton. Three man desgn concepts were appled to the confguraton. Frst, the njected CO 2 should be prevented Fg. 1. Co-rotatng twn screw extruder wth hgh pressure slt de extruson rheometer and metered CO 2 njecton system. Korea-Australa Rheology Journal June 2006 Vol. 18, No. 2 85

4 Sang Myung Lee, Jae Ho Han, Kyung Yl Km, Young Joon Ahn and Jae Wook Lee Fg. 2. Schematc of screw confguraton of a co-rotatng twn screw extruder. Fg. 3. A schematc of the slt de and nozzle. from leakng backward along the barrel. A reverse conveyng element was postoned rght before the CO 2 njecton to generate a melt seal and prevent CO 2 backflow. Second, the barrel pressure after the CO 2 njecton should be mantaned over the solublty pressure. The hgh pressure was generated by controllng the flow resstance at the nozzle attached to the slt de. The generated pressure level depends on the de openng area and the melt vscosty. Fnally, an extensve mxng of the polymer and CO 2 s essental for the complete dssoluton of supercrtcal CO 2 under a lmted resdence tme. The extensve mxng was acheved usng dedcated mxng elements. A schematc of the slt de and nozzle used n ths work s shown n Fg. 3. The nozzle s requred to elevate the pressure wthn the slt de above the bubble pressure ensurng that a one-phase mxture s mantaned durng measurement wth CO 2. The dmensons of the slt de and two nozzles are dsplayed n Table 1. The pressure drop and melt temperature across the slt de are measured va three pressure transducers (PT462E from Dynsco) and one temperature transducer. Table 1. The dmensons of the slt de and two nozzles Dmenson Slt de Nozzle 1 Nozzle 2 Length 65 mm(p 1 P 3 ) 38 mm 38 mm Wdth 15 mm - - Heght 2.5 mm - - Dameter mm 2.5 mm 4. Results and dscusson 4.1. Expermental verfcaton of rheometer desgn In order to determne the vscosty measurng condtons when CO 2 was njected, the avalable processng varables such as the resn feed rate and the screw speed, were tested. The barrel temperature was set to 145/170/175/180/180/ 180/180/180/180/180 wth the de at melts temperature of 180 o C. Pressures at the barrel and the slt de were montored at varous screw speeds and feed rates. The effects of each varable are descrbed n ths secton Screw speed Mantanng the hgh pressure durng the extruson wth 86 Korea-Australa Rheology Journal

5 Hgh-pressure rheology of polymer melts contanng supercrtcal carbon doxde Fg. 6. Mechansm for morphology change of polymer/scf system n the extruson process. Fg. 4. Relatonshp between pressures and screw speed durng extruson. Fg. 5. Relatonshp between pressures and feed rate durng extruson. CO 2 s crtcal because the njected CO 2 should be dssolved nto the polymer wthn a gven resdence tme. Therefore, the effect of the screw speed on the pressure was nvestgated for polypropylene/co 2 system, and the behavor at varous screw speeds s shown n Fg. 4. When the polymer feed rate was fxed at 5.1 kg/h, the barrel pressure consstently decreased wth ncreasng the screw speed, whle the pressures at the slt de remaned relatvely constant. Ths ndcates that, as expected, hgher screw speeds decrease the degree of fll due to the ncreased conveyng capablty Feed rate The pressure behavor wth the polymer feed rate was also nvestgated at 55 rpm. As expected, all pressures at the barrel and the slt de ncreased as the polymer feed rate ncreased, as shown n Fg. 5. It should be noted that the barrel pressure ncreases faster than the slt de pressure. As a result, low screw speeds and hgh polymer flow rates are recommended to mantan hgh pressures durng the extruson. Therefore, n ths wok, vscosty measurement as a functon of shear rate was carred out by the change of feed rate at constant screw speed Pressure profle at slt de under condtons wth and wthout CO 2 The vscosty of polymer/co 2 mxture must be measured at one-phase state. Mechansm for morphology change of polymer/supercrtcal flud (SCF) system n the extruson process shows n Fg. 6. After CO 2 njected nto barrel, t s mxed wth polymer and dffused nto polymer. Lastly, polymer and scco 2 wll be mantaned one phase by dffuson. Addtonally, a homogenous one-phase mxture must be present under fully developed flow condtons. In several publcatons on polymer melt rheology t s suggested that a length greater than 12 tmes the heght (H) s requred before fully developed flow s acheved (Laun, 1983; Leblanc, 1976). However, some expermental results have shown that an entrance length of upwards of 15H s requred (Rauwendaal et al., 1985). An entrance length of 15H has been used n ths study (see Fg. 3). The condton of fully developed flow was examned by measurng the pressures at three postons along the length of the slt de. Frst of all, pressure drop along slt de length of the neat PP was measured for varous feed rate at constant screw speed as shown n Fg. 7. The expermental data ponts for all condtons show a lnear pressure drop across the length of the slt de, respectvely. Fg. 8 shows pressure drop along slt de length of PP contanng CO 2 2 wt% as a functon of feed rate. Ther pressure drops shows lnear behavor as well as the results of neat PP. Ths means that one phase system was mantaned durng the measurements. In the vscosty measurement of PP/CO 2 system, CO 2 was njected nto the barrel up to 4 wt%. The pressure drop of PP contanng CO 2 4 wt% also shows lnear behavor, and all vscosty measurements were performed at operatng condtons over supercrtcal pressure of CO 2 (73.8 bar) Determnaton of densty for pure PP and PP/CO 2 mxture In order to obtan vscosty measurements at constant CO 2 concentratons, the estmaton of the mass flow rate s requred. To accomplsh ths estmaton, polypropylene s Korea-Australa Rheology Journal June 2006 Vol. 18, No. 2 87

6 Sang Myung Lee, Jae Ho Han, Kyung Yl Km, Young Joon Ahn and Jae Wook Lee Fg. 7. Pressure drop along slt de length durng vscosty measurement at screw speed of 55 rpm. Fg. 9. PVT data of polypropylene at coolng rate of 5 o C/mn. Fg. 8. Pressure drop along slt de length durng vscosty measurement at screw speed of 55 rpm. Fg. 10. The best ft of S-L EOS for PVT data over melt condton of polypropylene. Table 2. Characterstc parameters of pure components Substance P * T * (MPa) (g/cm 3 ) (K) Reference Carbon doxde (15) Polypropylene fed to the extruder va the hopper and the feed rate s used to generate pressure, forcng the melt nto the slt de. The pressure drop s recorded and samples of the melt are taken at the ext of the de to measure the mass flow rate. The volumetrc flow rate, whch s requred to determne the vscosty of the melt, s calculated usng the measured mass flow rate and the densty estmated from an equaton of state. For the purpose of ths analyss, the S-L EOS s used wth the mxng rules descrbed prevously (see Secton 2.2). To derve the S-L EOS for the polymer/co 2 mxtures, the characterstc parameters of the equaton for both the neat polymer and CO 2 were requred. Fg. 9 shows PVT data of the neat polypropylene n the range of 50 to 250 o C and 0.1 to 80 MPa at coolng rate of 5 o C/mn. The characterstc parameters of the S-L EOS were obtaned by fttng Eqs. 5~6 to the expermental data n the range from 0.1 to 50 MPa and from 120 to 250 o C as shown n Fg. 10. The resultng characterstc parameters of PP are lsted n Table 2. The characterstc parameters of CO 2 could be found n the lterature (Sato et al., 1999) and lsted n Table 2 together wth those for the neat PP. The nteracton parameter between the polymer and CO 2 was calculated from the solublty of CO 2 nto PP. The CO 2 -solublty data for PP was found n lterature (Sato et al., 1999). The solublty data n Fg. 11 was measured by a magnetc suspenson balance (MSB) from Rubotherm Inc. The nteracton parameter, δ 12, of PP/CO 2 system was expermentally obtaned by correlatng the solublty data wth S-L EOS. The calculated nteracton parameters were lsted n Table 3. The densty for PP/CO 2 mxture at ρ * 88 Korea-Australa Rheology Journal

7 Hgh-pressure rheology of polymer melts contanng supercrtcal carbon doxde Fg. 11. Solublty of CO 2 n the molten polypropylene. Fg. 12. Vscosty vs. wall shear rate at nozzle dameter of 1.5 mm and 180 o C. Table 3. Bnary nteracton parameter T ( o C) Bnary nteracton parameter, δ varous pressures and temperatures was estmated from S-L EOS and known nteracton parameter The vscosty measurement of the neat PP and PP/CO 2 mxture The vscosty of the neat PP and PP/CO 2 mxture were measured at varous shear rates and dfferent temperature for two nozzles. All vscosty measurements were performed at the condton wth lnear pressure drop profle as a functon of de length. They were carred out as a functon of feed rate at fxed screw speed of 55 rpm. The desred amount of CO 2, based on the mass flow rate calbraton, s njected by varyng the tenson on the back pressure regulator and the flow rate of the syrnge pump. The vscosty of polypropylene s shown n Fg. 12 wth CO 2 concentratons of 0, 1, 2, 3 and 4 wt% at 180 o C and nozzle 1. The vscosty s found to decrease wth ncreased ncorporaton of CO 2. The vscosty shear rate curve appears to preserve ts shape and shft to lower values. Ths trend of vscosty reducton has been observed not only for CO 2 addton (Han et al., 1983; Elkovtch et al., 1999), but for other plastczers as well (Gould, 1965; Rtche, 1972). Expermental measurement of vscosty reducton of PP for nozzle 2 at 180 o C wth the addton of pressurzed CO 2 s dsplayed n Fg. 13. The vscosty was found to decrease wth ncreased CO 2 concentraton at varous shear rates and two nozzles. Vscosty data of PP and PP/CO 2 mxtures at nozzle 1 showed generally hgher values compared to nozzle 2. Ths result s due to pressure effect. However, slopes of vscosty vs. shear rate were almost the same. Fg. 14 Fg. 13. Vscosty vs. wall shear rate at nozzle dameter of 2.5 mm and 180 o C. Fg. 14. Vscosty vs. wall shear rate at nozzle dameter of 1.5 mm and 200 o C. shows the vscosty curves of PP as a functon of CO 2 concentraton at 200 o C and nozzle 1. In comparson wth result Korea-Australa Rheology Journal June 2006 Vol. 18, No. 2 89

8 Sang Myung Lee, Jae Ho Han, Kyung Yl Km, Young Joon Ahn and Jae Wook Lee n Fg. 12, for temperature effects, CO 2 acts as an effectve plastczer, decreasng the vscosty wth ncreasng CO 2 concentraton. However, as temperature s ncreased the observed reducton n vscosty decreases slghtly. The data n Fgs. 12~14 confrm that CO 2 s an effcent plastczer for polypropylene, lowerng the vscosty of the polymer melt by 10-70% dependng on operatng condtons. 5. Conclusons A hgh-pressure extruson slt de rheometer was constructed to measure a wde range of vscostes for polymer melts plastczed wth dssolved carbon doxde. Nozzles added to the slt de rheometer were requred to elevate the system pressure above the bubble pressure of the polymer/ CO 2 mxture and thus mantan a one-phase soluton. Measurements of pressure drop confrmed the presence of a one-phase mxture and a fully developed flow n the system. Expermental measurements of vscosty as a functon of shear rate, pressure, temperature and CO 2 concentraton were conducted for commercal polypropylene. The CO 2 was shown to be an effectve plastczer for polypropylene, lowerng the vscosty of the polymer melt by as much as 70%, dependng of the process condtons and CO 2 concentraton. Ths s due to the reducton of chan-chan nteractons and ncreasng the nter-chan dstance. Acknowledgements Ths work was supported by the Korea Research Foundaton Grant funded by Korean Government (MOEHRD) (KRF D00001). Nomenclatures H M N N P P P * Q R R T T * v * W : The slt heght : Molecular weght : The total number of molecules n the lattce : The number of molecules : The measured pressure drops : The reduced pressure : The characterstc pressure : Volumetrc flow rate : The number of lattce stes occuped by a molecule of molecular weght : Gas constant : The reduced temperature : The characterstc temperature : The characterstc volume of a lattce ste : The slt de wdth Greek Letters γ ap : Apparent vscosty γ w δ j ε * η ρ ρ * τ w φ o φ : Wall shear rate : The bnary nteracton parameter : The nteracton energy : Vscosty : The reduced densty : The characterstc densty : Wall shear stress : The volume fracton of component n the mxture : The close-packed volume fracton of component n the mxture References Chou, J.S., J.W. Barlow and D.R. Paul, 1985, Plastczaton of glassy polymers by CO 2, J. Appl. Polym. Sc. 30, Elkovtch, M.D., D.L. Tomasko and L.J. Lee, 1999, Supercrtcal carbon doxde asssted blendng of polystyrene and poly (methyl methyacrylate), Polym. Eng. Sc. 39, Gould, R.F., 1965, Plastczaton and Plastczer Processes, Washngton, D.C. Han, C.D. and C.Y. Ma, 1983, Rheologcal propertes of mxtures of molten polymer and fluorocarbon blowng agent. I. Mxtures of low-densty polyethylene and fluorocarbon blowng agent, J. Appl. Polym. Sc. 28, Han, C.D. and C.Y. Ma, 1983, Rheologcal propertes of mxtures of molten polymer and fluorocarbon blowng agent. II. Mxtures of polystyrene and fluorocarbon blowng agent, J. Appl. Polym. Sc. 28, Lacombe, R.H. and I.C. Sanchez, 1976, Statstcal thermodynamcs of flud mxtures, J. Phys. Chem. 80, Laun, H.M., 1983, Polymer melt rheology wth a slt de, Rheol. Acta 22, Leblanc, J.L., 1976, New slt de rheometer: some results wth a butadene-styrene block copolymer, Polymer 17, Lee, M.H., C.B. Park and T. Tzoganaks, 1999, Measurements and modelng of PS/supercrtcal CO 2 soluton vscostes, Polym. Eng. Sc. 39, Lee, M.H., C. Tzoganaks and C.B. Park, 2000, Effects of supercrtcal CO 2 on the vscosty and morphology of polymer blends, Adv. Polym. Tech. 19(4), Park, C.B., A.H. Behravesh and R.D. Venter, 1998, Low densty mcrocellular foam processng n extruson usng CO 2, Polym. Eng. Sc. 38, Rauwendaal, C. and F. Fernandez, 1985, Expermental study and analyss of a slt de vscometer, Polym. Eng. Sc. 25, Rtche, P.D., 1972, Plastcsers, Stablsers and Fllers, Illffe Books Ltd., London. Royer, J.R., Y.J. Gay, J.M. Desmone and S.A. Khan, 2000, Hgh-pressure rheology of polystyrene melts plastczed wth CO 2 : Expermental measurement and predctve scalng relatonshps, J. Polym. Sc.: Part B: Polym. Phys. 38, Sanchez, I.C. and R.H. Lacombe, 1978, Statstcal Thermodynamcs of Polymer Solutons,GMacromolecules 11, Sato, Y., K. Fujwara et al., 1999, Solubltes and dffuson coeffcents of carbon doxde and ntrogen n polypropylene, hghdensty polyethylene and polystyrene under hgh pressures and temperatures, Flud Phase Equlbra 162, Korea-Australa Rheology Journal