Recycling Polyethylene from Automotive Fuel Tanks

Giuliana Gorrasi,1 Luciano Di Maio,1 Vittoria Vittoria,1 Domenico Acierno2

1Department of Chemical and Food Engineering, University of Salerno, Via Ponte don Melillo, 84084 Fisciano (SA), Italy2Department of Materials and Production Engineering, University of Naples, 80125 Naples, Italy

Received 22 March 2001; accepted 4 January 2002Published online 30 July 2002 in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/app.10967

ABSTRACT: Polyethylene (PE) from postuse automotivefuel tanks is considered a valuable material for mechanicalrecycling, and we assessed its properties, paying particularattention to transport properties and processability. Thecharacterization included the study of the sorption–desorp-tion isotherms, the rheological analysis of the molten mate-rials and their processability. In particular, we obtained,sorption-–desorption isotherms using a model molecule (n-heptane) to simulate contact with the fuel. The measure-ments were carried out on films of PE blend for tanks andseparately on the components of the blend. Rotational rhe-ometry was carried out on scraps from used tanks and onvirgin material for comparison. We performed some extru-

sion tests to evaluate the possibility of mechanical recyclingof the postuse materials. In particular, we examined themelt-fracture incoming conditions by making use of a twin-screw extruder with a round die. Stress–strain measure-ments were carried out on films of virgin and used material,obtained with a press on a laboratory scale to evaluate thechange of the mechanical properties of a manufacture ob-tained by reprocessing a polymer aged in contact with amixture of liquids. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci86: 347–351, 2002

Key words: polyethylene (PE); recycling; degradation

INTRODUCTION

In the last few years, there has been an increasingdemand, in terms of ecology, from car producers tofind new solutions to decrease the negative impact ofvehicles on the surrounding environment.

The possibility of recycling the major part of vehi-cles at the end of their useful lives is one of the mostdiscussed matters. Currently, much interest is focusedon the possibility of recycling the plastic components,which represent about 15% of the vehicle.

The recycling of plastics used from cars, besides beingimportant from an ecological point of view, providesmany economical advantages. The recovery operationsare not very difficult to carry out, principally becauseplastic components are easily recognized and separatedfrom the other parts when the vehicle components aredismantled and also because they could be reprocessedto obtain the same manufactures they were in their “firstlives,” in a closed-loop recycling.

Polymeric materials are applied in dashboards, roofcovers, and fuel tanks. Currently, there are some dataabout the possibility of recycling dashboards and roofcovers, mainly regarding the rheology of the usedmaterial and the mechanical properties of the new

manufactures,1,2 whereas few data are available onfuel tanks.3 The recycling operations, in the case of thefuel tanks, appear complicated by the fact that theplastic containers are used in contact with a mixture ofliquids composing the fuel, and some solvent remainsin the material even after many hours of drying undervacuum. This could cause problems and risks duringthe steps of regranulation and successive processing.

This investigation was a part of a wider projectbased on the possibility of recycling polymeric parts ofcivil and industrial vehicles. In this article, we presentthe results of absorption and desorption studies of ahydrocarbon molecule in a high-density polyethylene(HDPE) blend composing the fuel tanks.

We chose the hydrocarbon n-heptane as a modelmolecule to stimulate contact with the fuel. A simplemolecule whose vapor activity (a) could be easily var-ied between 0.1 and 1 could help clarify many impor-tant aspects of the problem.

We also performed rheological and mechanicalstudies to analyze the behavior of the used materialin the melt and also to investigate possible degra-dation phenomena that occurred in the materialreprocessed after a prolonged contact with the liq-uids. We thus simulated mechanical recycling byextrusion tests performed on a laboratory scale tomake a preliminary evaluation about the process-ability of tank scraps.

EXPERIMENTAL

All the materials were kindly supplied by FIAT(Torino, Italy).

Correspondence to: V. Vittoria (vvittoria@unisa.it).Contract grant sponsor: National Council of Research

(CNR)-Progetto Finalizzato Materiali Speciali per Technolo-gie Avanzatell.

Journal of Applied Polymer Science, Vol. 86, 347–351 (2002)© 2002 Wiley Periodicals, Inc.

HDPE, free of any additives, was molded in a hotpress into films of 0.15 mm thickness [polyethylene(PE) sample].

The virgin blend from unused fuel tanks, composedof HDPE and Carbon Black (CB; 0.2% w/w), wasmolded in a hot press into films of 0.15 mm thickness(PE–CB sample).

A blend of HDPE and CB obtained by granulationof fuel tanks from used cars was molded in a hot pressinto films of 0.15 mm thickness (Used (U)–PE–CBsample). CB powders were used as received.

Sorption–desorption curves were obtained by a mi-crogravimetric method at different vapor activity (a) (a� P/Po, where P is the actual pressure to which thesample was exposed and Po is the saturation pressureat the test temperature), according to a previouslyreported procedure.4 The model molecule used as apenetrant was n-heptane, and the experiments wereconducted at 30°C.

Rheological tests were performed with a parallelplate rheometer (Rheometrics ARES, USA) on the ma-terials from both used (U–PE–CB) and unused (PE–CB) fuel tank scraps. On the same materials, process-ability tests were carried out with a twin-screw ex-truder (Haake, Germany) at a temperature of 260°C.

Mechanical properties were tested for the PE–CBand U–PE–CB films with an Instron dynamometer(model 4301) (England) at room temperature. We ob-tained stress–strain curves by deforming the 10-mmspecimens at an initial rate of 10 mm/min and detect-ing the stress on the initial section.

All the experimental measurements were averagedon 3 samples; for the mechanical properties, we used10 samples.

RESULTS AND DISCUSSION

Sorption and desorption

Fuel tanks, during all of their lifetime, are in contactwith a liquid mixture of solvents, and each of them hasa specific interaction with the polymer. To simplify thestudy of the consequences of the contact of solventswith the tanks, we decided to perform a laboratorysimulation using n-heptane as model molecule. Wemonitored the sorption–desorption behavior in a widerange of activity, a � P/Po, between 0.2 and 1, todeeply investigate the solvent–polymer interactions ata controlled vapor pressure. In this way, it was possi-ble to plot the complete isotherm of sorption–desorp-tion, that could give useful information of any hyster-etic phenomenon.

Figure 1 shows the sorption–desorption isotherm ofn-heptane at 30°C in PE–CB. The sorption of the hy-drocarbon in the HDPE–CB blend was linear with a upto 0.6, and only at higher activities was a more thanlinear sorption, according to Flory–Huggins sorption

behavior,5 shown. The experimental desorption datashowed hysteresis of solvent sorption into the poly-mer in the whole investigated range of activity. Thehysteretic behavior was particularly relevant in thelow range of a, between 0.2 and 0.4. This means that anamount of solvent remained entrapped into the struc-ture after many hours of desorption under vacuum.

To separate the contribution of each component ofthe blend, that is, HDPE and CB, to the hysteresisphenomenon, we conducted separate experiments onthe two components. In Figure 2, the sorption–desorp-tion isotherm of n-heptane through PE is shown. It isevident that there was no hysteresis of sorption of thesolvent into the pure polymer; as a matter of fact, theamount of sorbed solvent at each a was totally rejected(in the experimental error range) after drying in thevacuum. Figure 3 displays the sorption–desorptioncurve relative to CB powders with n-heptane. Theshape of the plot calls for particular attention. Thesorption curve of n-heptane into CB showed a differ-ent sorption behavior, that is, a Langmuir-type iso-therm.5 We interpreted this isotherm assuming that,for low a, the solvent was absorbed on preferentialsites of the material, reaching a saturation level whenall the specific sites were saturated. This behavior wasalready found for n-pentane in CB powders.6

During the desorption cycle, a significant hysteresisof sorption of the n-heptane molecules in the powdersof CB was evident. This result is very important be-cause it shows that a high amount of solvent remainedentrapped into the CB structure on preferential phys-ical sites, and these molecules, firmly absorbed, were

Figure 1 (F) Sorption and (�) desorption isotherms ofn-heptane at 30°C for PE–CB.

348 GORRASI ET AL.

unable to be rejected even after many hours of desorp-tion under vacuum. With regard to the HDPE–CBblend this result could suggest that the observed hys-teresis of sorption was principally due to the presenceof CB in the PE matrix.

To verify the suggestion that the sorption hysteresisin the HDPE–CB blend, especially at low activity, was

due to the second component (CB), we prepared threeblends of pure PE and CB powders, with different CBcontents (w/w), 0.1, 0.2, and 0.4%, and investigatedthe sorption hysteresis. It is worth recalling that thepercentage of CB present in the blend with HDPE fortanks was reported by FIAT to be 0.2%.

Table I shows the values of residual n-heptane con-centrations in the three blends, after exposure to thevapor of n-heptane at a � 0.2 and subsequent desorp-tion in vacuum. The residual solvent increased onincrease of CB content. This result seems to confirmthat the hysteresis of sorption of n-heptane, used as amodel molecule, could be principally attributed to aspecific interaction with CB. This was only a qualita-tive indication of the influence of CB added to PE. Infact, the preferential sorption on the surface of theinorganic particles depends on the particle dimen-sions, their interactions with the polymeric matrix,and their distribution inside the plastic matrix: in thispreliminary study, we did not quantitatively investi-gate either of these characteristics.

Rheological behavior

The tests were performed with a rotational rheometerin dynamical mode in the range 0.01–10 Hz. Parallelplate geometry was used with plate diameters of 25mm. The gap was fixed at 2 mm, and the strain am-plitude was 20% at temperatures of 240 and 260°C. InFigure 4, the flow curves of the two types of materialsare shown. The results clearly show that the flowproperties of material from used tanks were quitedifferent, in particular at low deformation rates. Theused materials showed an evident increase of viscos-ity, which became less evident at higher deformationrates. Moreover, as highlighted from the flow curvesat different temperatures, there was a major tempera-ture effect on the rheological properties of U–PE–CBwith respect to PE–CB. Probably such a stiffening ofthe melt could be ascribed to crosslinking inducedboth from the residual solvent and the thermome-chanical processing at high temperatures that occursduring a rheological test. The effect of viscosity in-creasing during the process was highlighted by a tem-perature sweep test performed on scraps from used

Figure 2 (F) Sorption and (�) desorption isotherms ofn-heptane at 30°C; for PE.

Figure 3 (F) Sorption and (�) desorption isotherms ofn-heptane at 30°C for the CB powders.

TABLE IEquilibrium Sorption of n-Heptane at Vapor Activity

Equal to 0.2 and Successive Desorption UnderVacuum for PE–CB and the Blends with

Different Percentages of CB

Sample Ceqsorp (%) Ceq

des (%)

PE–CB 0.88 0.670.1% CB blend 0.47 0.340.2% CB blend 0.65 0.470.4% CB blend 0.50 0.12

RECYCLING PE FROM FUEL TANKS 349

tanks. The experiment, whose results are reported inFigure 5, was carried out at 240°C and at a frequencyof 10 Hz. In these conditions, the viscosity displayed asensible increase, which was in agreement with thevariation highlighted in Figure 4 between U–PE–CBand PE–CB at a given deformation rate.

Processability tests

To assess the suitability of tank scraps to be mechan-ically recycled, some extrusion tests were carried outat 240 and 260°C and at variable screw speeds. Ouraim was to have some preliminary indications aboutthe operating conditions and material behavior duringthe extrusion process. In particular, the screw speed atthe onset of melt fracture and the required torquewere recorded.

The results are reported in Table II. As expected, thematerial from used tanks (U–PE–CB) exhibited a

lower speed for the beginning of melt fracture, and ata fixed screw speed, a higher torque was necessary tooperate the extrusion. An increase in temperature,although not tested, could allow the operation athigher speed without the occurrence of melt fracture.However, a higher temperature could, in turn, alsosupport major thermal degradative and/or crosslink-ing effects. These results again seem to indicate thatalthough very slightly revealed from rheological tests,during the normal use of the material in contact withfuel, a structural modification takes place. In any case,extrusion is possible if one takes into account that alower speed and a reduced output must be chosen forthe operating conditions of the used material respectto the virgin one.

Mechanical behavior of films from virgin andrecycled scraps

In Figure 6, the stress–strain curves of the PE–CB andU–PE–CB films are shown. The two materials weremolded at 200°C on a laboratory scale. The two filmsshowed the typical behavior of a semicrystalline sys-tem, which deformed with neck propagation. At thebeginning, the stress was proportional to the deforma-tion, according to Hook’s law; after the yield point andthe drop after the yield, there was an interval of al-most constant stress, in which the neck, formed afterthe yield point, propagated to the whole sample. Afterthe neck propagation, we observed a zone in which abigger stress was needed for small deformations be-cause now we were drawing an oriented fibrous struc-ture, up to the breaking of the film. In Table III, wereport the mechanical parameters: elastic modulus [E(MPa)], stress at the yield point [�y [MPa)], stress atthe break point [�b (MPa)], the postyield stress drop[PYSD (MPa)], and the maximum elongation [�b (%)],derived by the stress–strain curves for both the sam-ples. The drawing curves and the mechanical datareported in Table III show that the film of used mate-rial (U–PE–CB) broke before and exhibited mechanicalproperties worse than the virgin one. Anyway, such aworsening was limited, and we can affirm that from apolymer aged in contact with fuel does not undergorelevant degradation phenomena.

CONCLUSIONS

The recycling process of fuel tanks from used carsdepends on the possibility of eliminating all the en-

Figure 4 Flow curves of (Œ) PE–CB and (‚) U–PE–CB at240°C and (�) PE–CB and (�) U–PE–CB at 260°C.

Figure 5 Viscosity of U–PE–CB as a function of time duringa rheological test at 240°C and 10 Hz.

TABLE IIExtrusion Test Conditions for the Beginning

of Melt Fracture

MaterialCritical ScrewSpeed (rpm) Torque (N m)

PE–CB 90 35U–PE–CB 60 45

350 GORRASI ET AL.

trapped solvent present, due to the prolonged contactwith liquid fuel, and on a careful analysis of the me-chanical behavior of the recycled materials.

Sorption–desorption investigations with a hydro-carbon model molecule showed that a possible sourceof sorption hysteresis can be considered as the prefer-ential interaction of the solvent molecules on specificsites of the CB particles.

From rheological analysis, we found differences be-tween used and unused materials in the molten state.In particular, it seemed that the presence of traces offuel, which could desorb during the thermal process-ing operation (and thus also during the melt rheologi-cal tests), could promote some structural modifica-tions of the polymer. Possible crosslinking between

the chains could stiffen the melt. These, in turn, couldmake necessary the use of different working condi-tions to perform mechanical recycling. However, thisoperation was still possible. The films obtained afterpressing the used material at 200°C on a laboratoryscale showed mechanical properties slightly worsethan the ones made of virgin polymer, which were,nevertheless, still acceptable.

References

1. Ragosta, G.; Musto, P.; Russo, P.; Zeloni, L.; Camino, G.; Luda,M. P.; Recca, A. Presented at Macromolecules ‘99, University ofBath, Bath, United Kingdom, 1999.

2. Ragosta, G.; Musto, P.; Martuscelli, E.; Russo, P.; Zeloni, L. J MatSci, 2000, 35, 3741.

3. Yernaux, J. M.; De Canniere, J. Presented at Recycle ‘95—Envi-ronmental Technologies, Davos, Switzerland, May 15–19, 1995.

4. Araimo, L.; de Candia, F.; Vittoria, V.; Peterlin, A. J Polym SciPolym Phys Ed 1978, 16, 2087.

5. Rogers, C. E. Physics and Chemistry of Organic Solid State;Interscience: New York, 1965.

6. de Candia, F.; Gargani, A.; Renzulli, A. Kautsch Gummi Kunsts1991, 44, 124.

Figure 6 Stress–strain curves for PE–CB and U–PE–CB.

TABLE IIIMechanical Parameters Obtained from the Stress–Strain

Curves Reported in Figure 6

Sample E (MPa) �y (MPa) �b (MPa) PYSD (MPa) �b (%)

PE–CB 297.73 10.25 13.30 2.40 6.70U–PE–CB 263.18 7.95 11.10 2.00 5.60

RECYCLING PE FROM FUEL TANKS 351