Degradation of post-consumer polypropylene materials exposed tosimulated recycling—mechanical properties

Anna Jansson*, Kenneth Moller, Thomas Gevert

SP Swedish National Testing and Research Institute, Department of Chemistry and Materials Technology,

PO Box 857, SE-501 15 Boras, Sweden

Received 6 March 2003; received in revised form 11 April 2003; accepted 12 April 2003

Abstract

Two post-consumer polypropylene (PP) materials have been examined and compared using a simulated recycling test procedureconsisting of repeated cycles of alternating extrusion and accelerated thermo-oxidative ageing. The materials were also subjected to

repeated extrusion and thermo-oxidative ageing separately. The materials were characterized by mechanical and chemical proper-ties, such as elongation-at-break, oxidative induction temperature, carbonyl index and changes in molecular weight distributions.The different batches of recycled PP differed substantially in terms of durability and recycling potential. This illustrates one of theproblems involved in handling recycled materials i.e. different batches and material streams may differ quite a lot. Interestingly, the

elongation-at-break was significantly different for samples taken after ageing and extrusion, respectively. The elongation decreasedupon each ageing step, and increased as a result of the subsequent extrusion. Furthermore, the combination of extrusion and ageingtended to degrade the materials faster compared to ageing or repeated extrusion performed separately.

# 2003 Elsevier Ltd. All rights reserved.

Keywords: Polypropylene; Recycling; Degradation; Tensile testing; DSC; FTIR; SEC

1. Introduction

The large amount of waste produced by society is agrowing environmental problem that has to be managedin order to achieve a more sustainable society. One wayto reduce waste is to recycle materials such as paper,glass, metals and polymers in closed loop systems,where the same materials are used several times. In thesetypes of systems, the materials are repeatedly collected,reprocessed, and then shaped into new products i.e.mechanical recycling. Whereas paper, glass and metalsare today extensively recycled by mechanical recyclingin Sweden, according to the packaging collection ser-vice, only a small percentage of polymeric packagingmaterials are recycled by re-melting and shaping intonew products according to the APME, Association ofPlastics Manufacturers in Europe [1]. This is togetherwith economical and logistic reasons, due to lack ofknowledge about quality and durability of recycled

polymeric materials. The fact that polymeric materialsare organic matter that degrade through oxidation dur-ing use and when the material is subjected to high tem-perature and shear forces when processed, can alsoexplain the limited recyling of these materials.Although some polymeric products are today recycled

by mechanical recycling, the majority is still put in alandfill, incinerated or otherwise disposed of. Further-more, products made from recycled polymers are oftencheap, of only moderate quality and of very simpledesign. Since extraction of raw material is the mostenvironmentally affecting part of polymer production, itwould be beneficial for the environment if recycledpolymeric material to a greater extent could replacevirgin material [2]. However, further knowledge aboutthe properties of recycled polymeric materials is neededin order to find appropriate and useful applications andincrease the use of these materials. One appropriateinvestigation procedure for polymer materials would bethe simulated recycling method, developed at our insti-tute (Fig. 1). In this method, the material is exposed toboth processing, simulating manufacturing of a product,and accelerated thermo-oxidative ageing, simulating the

0141-3910/03/$ - see front matter # 2003 Elsevier Ltd. All rights reserved.

doi:10.1016/S0141-3910(03)00160-5

Polymer Degradation and Stability 82 (2003) 37–46

www.elsevier.com/locate/polydegstab

* Corresponding author. Tel.: +46-33-165329; fax: +46-33-10-

3388.

E-mail address: anna.jansson@sp.se (A. Jansson).

usage phase. We have in previous studies used thesimulated recycling method to investigate High- andLow Density Polyethylene [3, 4], as well as an ABS(Acrylonitrile Butadiene Styrene) material [5]. For theinvestigated stabilised Low Density Polyethylene(LDPE) material, the combination of processing andaccelerated ageing seemed to degrade the material fasterthan processing or accelerated ageing performed sepa-rately. The material could withstand at least twice asmany separate extrusion or ageing steps compared tothe number of simulated recycling steps before theelongation at break reached a value below 100%, andthe material was considered degraded [3]. On the otherhand, the High Density Polyethylene (HDPE) materialwas not significantly affected even after 10 simulatedrecycling steps [4].Accelerated thermo-oxidative ageing is a common

method for estimating the service lifetime of polymericmaterials. Repeated extrusion or injection moulding, onthe other hand, are most often used to estimate their re-cycling potential [6–11]. In this study, two post-consumerpolypropylene (PP) materials have been investigated bycombined processing and ageing, as well as ageing andrepeated extrusion. Since PP contains tertiary hydrogenbonds, the material is more vulnerable to thermo-oxi-dative degradation than polyethylene.

2. Experimental

2.1. Material

Two different PP materials were recollected from realwaste streams and investigated. Both materials con-tained organic blue pigment (Cu-phtalocyanine accord-ing to the supplier) and had been used as storage boxesprior to re-collection and granulation into flakes. There-collection was done by Strandplast, Perstorp, Swe-den. The materials were chosen because they were quitewell defined regarding age and origin. Both materialswere received as ground flakes with an approximate sizeof 3�3�1 mm. The materials will from now on bedesignated PP-1 and PP-2.

The PP-1 material has been in service as storage boxesfor approximately 5 years, and was originally manu-factured by Borealis. It is a MC1312M grade co-poly-mer, containing small amounts of ethylene units toimprove the impact strength of the material. The MeltMass Flow Rate (ISO 1133) was 12-13 g/10 min (230 �C,2.16 N) according to the supplier. Two antioxidants,Tinuvin 770 (Hindered Amine Light Stabliliser, HALS)and Irgafos 168 (phosphite co-stabiliser) were found inthe PP-1 material using Time-of-Flight Secondary IonMass Spectrometry (TOF-SIMS) analysis. No quantifi-cation of the antioxidant content was done.The PP-2 material has been in service for approxi-

mately 10 years, was originally produced Borealis andhad a Melt Mass Flow Rate of 12–13 g/10 min at 230 �Cand 2.16 N, according to the supplier. The PP-2 mate-rial was also co-polymerised with a few percent of eth-ylene to achieve better ductility. This PP batchcontained differently coloured flakes, which turned outto be a contamination of approximately 1.2% by weightHDPE. Fourier Transform Infrared (FTIR) spectro-scopy was used to identify the contaminant material,which probably originated from the milling process.Antioxidants used in the PP-2 material, were identifiedby extraction and Time of Flight Matrix Assisted LaserDesorption Ionisation Mass Spectrometry (TOF-MALDI-MS) analysis. Irganox 1010 (hindered phenol)and Irgafos 168 (phosphite co-stabiliser) were detected,but no quantifications of the antioxidants were done.

2.2. Processing and ageing

Both PP materials were processed in a BrabenderPlasti-Corder PLE 651 Extrusiograph, single screwextruder. Screw dimensions were 19 mm diameter and500 mm length. A slit die with the dimensions 20.0�0.8mm was mounted on the extruder. The temperatureprofile was 150–175–200–220–230 �C and the screwrotation speed was 30 rpm for both materials. Theextruded films were cooled in air and gently stretched ona conveyor belt. The thickness of the extrudates wasapproximately 0.2 mm. In order to extrude the materialsseveral times and simulate closed loop recycling, thefilms were ground into flakes in a Rapid granulator1540 after each ageing step. Accelerated ageing wasperformed in Salvis draft air ovens at 130 �C. The filmsamples were neither in contact with the oven walls noreach other during the ageing. For PP-1, each ageing stepwas 48 h at 130 �C, which corresponds to at least 10years of indoor use. The estimation is based on theassumption that the thermooxidative ageing corres-ponds roughly to a doubling of the ageing rate for eachincrease of 10 �C. Dixon recommends the use of a rela-tively low activation energy when calculating the accel-erating factor in order not to overestimate the servicelife [12]. For PP-2, each ageing cycle was 24 h at 130 �C.

Fig. 1. The simulated recycling model. Samples are taken after each

ageing step as well as after each processing i.e. extrusion step.

38 A. Jansson et al. / Polymer Degradation and Stability 82 (2003) 37–46

The first PP material, PP-1, was subjected to threedifferent series of processing and/or ageing. In thesimulated recycling series (series SR1), PP-1 was sub-jected to alternating extrusions and ageing steps (48 h at130 �C). Ten extrusions and nine ageing steps were per-formed. In the second series (series EA1), ten extrusionswere performed, and then the material was aged for 480h. Finally, in the third series (series AE1), the materialwas aged for 480 h prior to 10 repeated extrusions. Notethat step one in the AE series is an extrusion step toform the strips from which samples are taken. The pur-pose of these investigations was to verify if alternatingextrusion and ageing degrade polymer materials fasterthan either procedure performed separately. Therefore,the PP-1 material was subjected to the same numbers ofextrusions and ageing steps in all three series, but indifferent combinations.Also the PP-2 material was exposed to three different

series of processing and/or ageing. In the first series(series A2), an extruded PP film was exposed to ovenageing at 130 �C for totally 480 h and samples weretaken out for analyses every 24 h. Note that step one inthe A2-series is an extrusion step. In the second series(series E2), the PP material was extruded 14 consecutivetimes. After each extrusion, samples were taken out foranalyses. The third series (series SR2), was a combi-nation of alternating extrusion steps and ageing steps(24 h at 130 �C), i.e. simulated recycling. Six extrusionswere performed, and samples were taken out after eachextrusion as well as after each ageing step.

2.3. Testing

Mechanical properties, i.e. elongation-at-break, werestudied by tensile testing according to the ISO-527standard. An Instron tensile tester 5566, equipped withan Instron 2663-302 video extensiometer was used.Dumb-bell shaped test specimens (SIS 162202) werepunched out from the extruded films and conditioned at23 �C and 50% relative humidity for 12 h before testing.Each reported result is an average of five to 12 tensiletests.Oxidative induction temperature (Tox) was measured

using either a Mettler Differential Scanning Calori-meter, DSC type 30 - TA 8000 or a TA InstrumentsDSC 2920. For all measurements, a temperature scanfrom 50 to 250 �C and a heating rate of 10 �C per min-ute were used. Samples weighing approximately 5 mgwere punched out of the films and put in aluminiumcrucibles. Tox was measured in oxygen atmosphere witha gas flow of 50 ml/min. Nitrogen was used as purge gaswhen measuring the degree of crystallinity. For all PP-1samples, the equipment from TA Instruments was used.The Mettler equipment was used for the PP-2 material,except for the last measurements in the repeated extru-sion series (extrusion step no. 11–14). Furthermore,

some comparing tests were made on PP-2 to make surethat the results from the two different instruments cor-responded.Carbonyl content in the degraded material was mea-

sured with a Cygnus 100 Mattson FTIR-spectro-photometer. The ratio between the absorbance of theintensity of the carbonyl peaks absorbing at 1712 cm�1

(carboxylic acid) and at 1720 cm�1 (ketonic carbonylgroup) and that of a reference peak (absorbing at 1892cm�1) was calculated. The ratio was used as a measureof the carbonyl content in the sample, here designatedCarbonyl Index (CI).Changes in molecular weight and molecular weight

distributions (Mw, Mn, and Mw/Mn) were measured bySize Exclusion Chromatography (SEC). The sampleswere dissolved in 1,2,4-trichlorobenzene before injec-tion. The instrument, a Waters C+V operated at135 �C, was calibrated with polystyrene standards forpolyethylene via the universal calibration curve. Totransfer to polypropylene calibration the data were cor-rected according to the two Mark Houwink relationsgiving a correction factor of 1.3 [13].TOF-SIMS analysis was used to determine the type of

antioxidant in the PP-1 material. The TOF-SIMS datawas recorded on a TOF-SIMS IV instrument (ION-TOF GmbH, Munster, Germany) using a pulsed pri-mary ion beam of 25 keV Ga+ at a current of 0.55 pA.The analysis area was between 200�200 and 300�300mm2 and the analysis time was 200 s. Positive spectrafrom both the sample surface and a freshly microtomedcross section surface were recorded.The antioxidants in the PP-2 material were extracted

in chloroform and analysed by TOF-MALDI-MS. ABruker Biflex III instrument with a detection limit of10�9–10�15 moles was used for the analysis. Extract (50ml) was mixed with 50 ml 0.15 M matrix solution i.e.Dithranol (1,8,9-Trihydroxynaphtalene) in Chloroformand 1 ml 0.15 M Chloroform–Sodium Chloride-solution.The accelerating current was 19 kV, and 25 scans wererun in order to achieve the spectra.

3. Results and discussion

3.1. The first polypropylene material, PP-1

3.1.1. Elongation-at break , PP-1Fig. 2 shows how the elongation-at-break varies dur-

ing simulated recycling (series SR1). One step in Fig. 2corresponds to either an extrusion or an ageing step (48h at 130 �C). After each ageing step the elongation-at-break dropped considerably, but returned more or lessto the initial value after extrusion. However, after thelast extrusion, the material was highly degraded andstrips could no longer be formed from the now waxlikematerial. Therefore, the elongation value was set to zero

A. Jansson et al. / Polymer Degradation and Stability 82 (2003) 37–46 39

for step 19 (Fig. 2). This type of zig-zag shaped curvehas been observed before, in a LDPE material subjectedto simulated recycling [3]. The behaviour of the elonga-tion data is still not fully understood but possibleexplanations are:

� Re-organisation of the lamellar structure; lamel-lar thickening during ageing at high temperature(130 �C) leads to a decrease of the elongation-at-break [14]. Re-extrusion and melting of thematerial include forming of new crystallites, withinitial size and structure.

� Surface degradation; predominantly the surfacelayer is degraded upon ageing. The degradationproducts act as stress raisers and lower theelongation-at-break values. In the subsequent re-extrusion a new surface layer is formed and theelongation values increase. Ageing of thin films in18O2 labelled oxygen, followed by TOF-SIMSanalysis has shown that the oxygen concentrationis higher in a very thin surface layer, probablywell below 1 mm down to a few nm, compared tothe bulk material [15].

� Dilution of degraded polymer chains; the oxida-tive degradation takes place in the amorphouspart of the material, since the oxygen diffusioninto the crystallites is very limited. As the inves-tigated material is PP, the degradation will givechain scission in the amorphous part. During re-processing, non-degraded material from thecrystalline phase is melted and blends with thedegraded polymer chains. Hereby, the degradedpolymer chains from the amorphous part are

diluted and more high molecular weight polymerchains become available to form tie moleculesbetween the crystallites. This may explain theimprovement of the elongation-at-break after theextrusions. When the melt is cooled and re-crys-tallisation occurs, both degraded and non-degraded polymer chains form crystallites.

In order to investigate the influence of the high ageingtemperature on the elongation-at-break, while avoidingoxidation, some PP-1 strips were treated thermally in anitrogen atmosphere at 130 �C for 48 h. Fig. 3 showsthat the thermal treatment in nitrogen had almost thesame effect on the elongation-at-break as ageing in cir-culating air. Furthermore, DSC measurements show anincrease in degree of crystallinity after ageing steps, andthis indicates that some re-crystallisation occurs duringthe ageing steps. A subsequent extrusion of the material,thermally treated in nitrogen, caused a recovery of theelongation-at-break value. A second thermal treatmentin nitrogen atmosphere again decreased the elongation-at-break value (Fig. 3). For comparison, reprocessingby compression moulding was studied. Only a few per-cent improvement of the elongation-at-break could beobserved after compression moulding. The mixing pro-cess in the extruder seems to be important for therecovery.In the AE1 series, nine ageing steps were followed by

ten extrusion steps. Samples were taken out every 48 hduring ageing and after each extrusion. The first 10steps in Fig. 4 show how the elongation-at-breakdecreases continuously during the ageing, from 700 toapproximately 50%. These data indicated a severe

Fig. 2. Elongation-at-break values for PP-1 subjected to simulated

recycling, series SR1. Step 1 is the first extrusion, step 2 the first age-

ing, step 3 the second extrusion, and so forth. After each extrusion

step the elongation values are in the range 600–700% and after each

ageing step the elongation drops, and a zig-zag shaped curve is

formed.

Fig. 3. Comparison of elongation-at-break for samples (PP-1, simu-

lated recycling SR1) aged at 130 �C in air and nitrogen respectively.

The broken line represents the material thermally treated in nitrogen.

For both types of ageing, the elongation-at-break drops after ageing

(step 2 and 4) and increases after the subsequent extrusion (step 3 and

5), indicating that oxidative degradation alone does not cause the

elongation drop.

40 A. Jansson et al. / Polymer Degradation and Stability 82 (2003) 37–46

degradation of the material. However, upon grindingand re-extrusion of the aged material the elongation-at-break value increased to almost the initial value. This isthe same type of increase in elongation-at-break thatwas observed upon each extrusion step in the simulatedrecycling experiments (Fig. 2). Interestingly, PP-1exhibited a final elongation-at-break value of 550% andcould therefore not be regarded as degraded even afternine steps of ageing followed by 10 extrusions (Fig. 4).In comparison, PP-1 was highly degraded after 19 stepsof simulated recycling. Evidently, the simulated recyclingdegrades the material faster.Fig. 5 shows the EA1 series where PP-1 was first

extruded ten times, and then aged for a longer period oftime. The elongation decreased slightly as a result of eachextrusion. After the first ageing step, the elongation

dropped to approximately 150% and remained at thatlevel until step 14, upon which it dropped to less than100%. After having been subjected to all 19 steps, thematerial was extruded once again to investigate if theinitial elongation-at-break value could be recovered.However, both the surface and the bulk material wereso degraded that the elongation could not be improved.Both extrusion and ageing degrades polymeric mate-

rials. The present investigation clearly shows that theorder in which extrusion and accelerated ageing areperformed is important. Simulated recycling and extru-sion followed by ageing degraded the material fastercompared to ageing followed by extrusion. One expla-nation to the faster degradation in the SR and EA seriesis hydroperoxide formation and decomposition.Hydroperoxides (ROOH) are intermediate degradationproducts formed upon polymer oxidation. They arerelatively stable at temperatures below 150 �C, butdecompose to form two highly reactive radicals at highprocessing temperatures. These radicals, i.e. alkoxy(RO.) and hydroxy (.OH) radicals, can then abstracthydrogen atoms from the polymer chains. Alkyl radicalsare thereby formed, and the auto-oxidation continues.The formation of the alkoxy and hydroxy radicalsaccelerates the oxidation process considerably, sincetwo additional radicals are formed out of one in eachoxidation step, according to the polyolefin autooxida-tion mechanism [16]. In series SR1 hydroperoxide for-mation and decomposition in every cycle may explainthe faster degradation. Since we have investigated apost-consumer material, hydroperoxides are most likelypresent in the starting material and they may decom-pose already in the first extusion step. Moreover, inseries EA1 imperfections caused by the extrusions mayaccelerate the degradation during the subsequent age-ing. When all the ageing steps were performed first, ser-ies AE1, the material could withstand nine steps ofageing followed by 10 extrusion steps. Probably, thefilm surface was slightly oxidised during the ageingwhereas the bulk material was substantially less degra-ded than would be expected for such a thin film (0.2–0.3mm). The following extrusion steps should furtherdegrade the material, but due to the presence of residualstabiliser, the material recovered after mixing in theextruder.In Fig. 6 all three series (SR1, AE1 and EA1) are

plotted in the same graph. The filled circles represent thesamples analysed after extrusion steps and the open cir-cles represent the samples analysed after ageing. Thestraight lines are obtained by least square linear fits forfilled and open circles respectively. The elongation-at-break values decrease continuously in both data series.The decrease in elongation-at-break noticed for thesamples analysed after ageing (open circles) represents amore or less reversible deterioration of physical nature,since the material recovers after a subsequent extrusion

Fig. 4. Elongation-at-break for PP-1, series AE1, ageing followed by

repeated extrusion. The ageing causes a decrease in the elongation-at-

break and the following extrusion improves the elongation. Step 11

represents the first extrusion step.

Fig. 5. Elongation-at-break for PP-1, series EA1, repeated extrusion

followed by ageing. The elongation slowly decreased upon extrusion.

On the subsequent ageing, step 11, the elongation dropped con-

siderably. After step 19 the elongation-at-break was only 5%.

A. Jansson et al. / Polymer Degradation and Stability 82 (2003) 37–46 41

step. However, the slope of the bottom straight line mayindicate a superimposed irreversible deterioration ofchemical nature. When the antioxidants are deactivatedno recovery of the material could be observed and thematerial fails, step 18 and 19 in cycles SR1 and EA1.

3.1.2. Degree of crystallinity, PP-1The degree of crystallinity was measured by DSC for

PP-1, series SR1, after a few extrusions and ageingsteps, respectively. Since the thermal history of thesamples, i.e. extrusion and cooling in air, compared tooven ageing at 130 �C was of interest, the crystallinitywas analysed without pre-heating. Table 1 shows thedegree of crystallinity for samples taken after extrusion(step 1, 3 and 15) and after ageing (step 2, 4 and 16) inseries SR1. The degree of crystallinity increased by 4–8% or 10–20% in a relative sense, after each ageingstep. The results were the same for the samples treatedthermally in nitrogen atmosphere. For these samples,surface oxidation should not play an important role forthe elongation-at-break results. These results indicatethat re-crystallisation could, at least partly, explain thezig-zag shaped elongation-at-break curve obtained fromthe simulated recycling experiments. In Fig. 3, elongation-

at-break for samples taken after ageing and after ther-mal treatment in nitrogen are compared. It seems likeageing in circulating air causes a larger drop in elonga-tion-at-break and the recovery after the subsequentageing is not as high as for the samples taken afterthermal treatment in nitrogen. Beside the re-crystal-lisation some degradation also seems to occur if oxygenis present during the ageing.

3.1.3. Oxidative induction temperature (Tox), PP-1Both ageing and extrusion cause antioxidant deacti-

vation, which gives decreased Tox values. In Fig. 7, Tox

values obtained during simulated recycling, series SR1,are compared to those measured upon 10 ageing stepsand then after repeated extrusions and those in oppositeorder, series AE1 and EA1, respectively. From step oneup to step nine the Tox values are quite similar for allthree series. From step 10 up to step 18 the three seriesdiffer more, probably due to in-homogeneous degrada-tion. In step 18 the Tox values are almost the same forall three series, but in step 19, Tox for the SR1 sampledecreases considerably, whereas the other two samplesremain on the same level. Tox for unstabilised poly-propylene has been measured to approximately 180 �Con soxhlet extracted PP material. In series AE1 the Tox

value drops approximately 8 �C after the first extrusion,step 11, Fig. 7. This can be explained by hydroperoxideformation during the thermooxidative ageing. At thehigh extrusion temperature these hydroperoxidesdecompose forming highly active radicals which rapidlydeactivates the antioxidants. Karlsson and Camachohave observed rapid initial decrase in oxidation induc-tion time (OIT) for PP upon repeated extrusion [17].

3.1.4. Carbonyl Index (CI) , PP-1The Carbonyl Index was measured for some of the

data points in the PP-1 material. A small carbonyl peakwas observed already in the starting material, probably

Fig. 6. The elongation-at-break for all three PP-1 series shown in the

same graph. The filled circles represent samples taken after the extru-

sion steps and the open circles, samples taken after ageing steps.

Table 1

Degree of crystallinity after extrusion and ageing respectively in series

SR1

Sample

Degree of

crystallinity (%)

Step 1 (after extrusion)

39

Step 2 (after ageing)

47

Step 3 (after extrusion)

41

Step 4 (after ageing)

46

Step 15 (after extrusion)

41

Step 16 (after ageing)

45 Fig. 7. Tox for PP-1, series SR1 (*), series AE1 (&) and series EA1

(~). Note that not all samples are measured in series AE1.

42 A. Jansson et al. / Polymer Degradation and Stability 82 (2003) 37–46

originating from additives or contaminants in thematerial. The carbonyl absorbance was in the samerange as the absorbance of the reference peak at 1891cm�1, resulting in a CI value of approximately 1, for thestarting material. A pronounced increase in the CI valuewas not detected until the material was very degraded,i.e. when the elongation-at-break was close to zero andthe Tox value had decreased to approximately 180 �C.Fig. 8 shows how the CI increases in step 19 in seriesSR1. Also the Tox value decreases at this point.In the AE1 series no increase in CI was detected in the

last step i.e. nine ageing steps followed by eleven con-secutive extrusions. However after the last ageing, step9, (18 days of ageing) degraded brittle spots wereobserved on the films. CI for a sample taken from adegraded area was 1.7 and CI for the sample taken froma non-degraded part of the strip was 1, similar as for thestarting material. This observation favours the dilutiontheory, where the degraded parts of the material aremixed with non-degraded material in the extruder. Thisobservation also illustrates the inhomogeneous natureof degradation in polymer materials.Ten consecutive extrusions in the EA1 series did not

affect the CI value. In step 19, after both extrusions andageing the extruded strips were partly degraded. The CIvalues differed depending on where the samples weretaken. The sample from a brittle part of the strip had aCI value of 2.5 while the CI for a non-degraded samplewas 1. The same phenomenon was observed for the Tox

analysis, the results depended on where the sample wastaken. Where the strip was degraded the Tox value wasonly 183 �C. The corresponding Tox value for a non-degraded part it was 198 �C, as showed in Fig. 7.

3.2. The second polypropylene material, PP-2

For comparison a second PP material, PP-2, wasinvestigated. PP-2 was differently stabilised, had been in

service for a longer period of time and was also con-taminated by HDPE. Three different test series wereperformed; simulated recycling (series SR2), repeatedextrusion only (E2) and ageing only (A2).

3.2.1. Elongation-at-break, PP-2Fig. 9 shows that the elongation-at-break for PP-2

was not considerably affected by 20 consecutive days ofageing at 130 �C (series A2). The material could alsowithstand 14 repeated extrusions with a drop in elon-gation-at-break from 800 to 400% (series E2). The PP-2material showed an opposite elongation-at-break beha-viour, compared to the PP-1 material. Ageing did notcause any significant drop in elongation-at-break for thePP-2, whereas repeated extrusion decreased elongation-at-break somewhat. One explanation for this oppositebehaviour could be the different stabilisation systems inthe two materials. The PP-1 material is stabilised with ahindered amine (Tinuvin 770) and the PP-2 materialcontains phenolic antioxidant (Irganox 1010). Bothmaterials also contain organic phosphite (Irgafos 168)as co-stabiliser. Moreover, HDPE flakes were found inthe PP-2 material and this impurity may explain thedifference in elongation-at-break between the twomaterials.The results from the simulated recycling are not as

straightforward as for the separate ageing or extrusion.Series SR2 shows a large spread in elongation-at-break.Each data point represents an average of at least fiveand most often more measurements. Even though thehighest and lowest values are excluded from the calcu-lations the standard deviations are still higher than100%. Large drops in elongation-at-break were

Fig. 8. Tox (&) and Carbonyl Index (*) for the PP-1 material plotted

in the same graph.

Fig. 9. Elongation-at-break for the PP-2 material; SR2 (*), A2 (&)

and E2 (~). A pronounced drop in elongation-at-break is observed

after step 6 and 9 in the SR2 series. After these drops the material

recovers again. Interestingly, the material can withstand 14 repeated

extrusions (E2) and 20 ageing steps (A2) with maintained elongation

at break values. In the simulated recycling series (SR2) the material

had an elongation-at-break value close to 0 and could not be recov-

ered by extrusion after step 12.

A. Jansson et al. / Polymer Degradation and Stability 82 (2003) 37–46 43

observed after step six and nine. After each of theseextremes, the material recovered and the elongationincreased to almost the initial value, and thereafterdecreased again. It is remarakble that the elongation-at-break drop observed in step nine occured after anextrusion step. In previous observations elongation-at-break drops have been observed after accelerated ageingsteps. One explanation is that this sample was takenfrom a more contaminated part of the material. Theobserved large spread in the results is probably due toan in-homogeneously degraded starting material. Sev-eral studies by staining as well as chemiluminescencetechniques have shown that oxidation of polyolefins is aheterogeneous process [18–24]. In addition, the con-tamination by HDPE contributes to the heterogeneousnature of PP-2. The results in Fig. 9 suggest that theinvestigated polypropylene material should not be recy-cled more than twice (two closed loops, or four steps inFig. 9), to guarantee the quality of the material in termsof elongation-at-break. After step 12, i.e. six extrusionswith intermediate ageing, the material was highlydegraded and could not be extruded into strips. There-fore, the elongation-at-break value was set to zero atthis point. Interestingly, the material seemed to degradefaster when subjected to simulated recycling, i.e. thematerial in the extrusion and ageing series consistentlywithstood more steps without decreased elongation-at-break values. Importantly, if PP-2 had been evaluatedby repeated extrusion only, the material would haveseemed to be able to withstand ten recycling steps.

3.2.2. Oxidative induction temperature (Tox) andCarbonyl Index (CI), PP-2Fig. 10 shows the results from the Tox measurements.

When exposed to ageing at elevated temperature thematerial lasted for 14 days without any drop in Tox

value. Repeated extrusion, on the other hand, lead to

rapid deactivation of antioxidant, i.e. Tox decreasedsubstantially already upon the five first extrusions,which was not the case for the simulated recycling series(series SR2). In order to investigate whether this was aneffect of in-homogeneity of the material, 10 films extru-ded once, but at different occations, were analysed. Themean value of Tox based on the measurements on these10 films was 190.7 �C, with a standard deviation of 2.9.The highest and lowest values were 194.7 and 185.6 �C,respectively, giving a temperature range of 9.1 �C. Thisclearly shows that the material is heterogeneous whichmight explain the Tox results in Fig. 10. Hence, theanomalies in the results are due to a below averageconcentration of stabiliser in the particular samplestudied. This is a quite common problem in degradedand recycled materials [25, 20]. No significant change indegree of crystallinity could be detected by DSC mea-surements after six simulated recycling steps.When the antioxidants are deactivated, oxygen reacts

with the polymer chain and forms carbonyl groups,mainly ketones and carboxylic acids. Carbonyl peakswere observed at �1712 cm�1 (carboxylic acid) and�1720 cm�1 (ketonic carbonyl group) for PP-2 whenexposed to simulated closed-loop recycling. The twopeaks are overlapping, but as the material degrades themaximum peak shifts towards the carboxylic acid band.In the PP-2 samples subjected to either ageing (seriesA2) or repeated extrusion (series E2), only very lowcarbonyl absorption could be detected by FTIR mea-surements. Hence, the CI value did not change duringthe oxidation of the polymer.When the Tox decreases, the carbonyl IR-absorbance

increases in the subsequent cycle in the SR2 series(Fig. 11). In other words, when all antioxidants hadbeen deactivated, carbonyl groups were rapidly formedby auto-oxidation, both through chain propagation andtermination reactions. At this stage of degradation the

Fig. 10. Tox for the three PP-2 test series, SR2 (*), A2 (&) and E2

(~). In the A2 and E2 series not all samples are analysed.

Fig. 11. Tox (&) and Carbonyl Index (*) for the PP-2 material are

plotted in the same graph. The drop in Tox is detected one cycle before

the corresponding increase in Carbonyl Index.

44 A. Jansson et al. / Polymer Degradation and Stability 82 (2003) 37–46

mechanical strength of the material was very poor.Since only small amounts of carbonyl groups could bedetected in either the extrusion or the ageing series, theantioxidants in these materials were probably not fullydeactivated. However, the Tox measurements showed adecreased antioxidant activity for both the extrusionand the ageing series, after the 9th and the 13th steps,respectively (Fig. 10). In accordance with the CI results,the elongation-at-break for series E2 and A2 were at alevel of 400–800% for the last cycles, which indicatesthat these samples still have acceptable mechanicalproperties.

3.2.3. Molecular weight distributions PP-2After the fourth extrusion step in the simulated recy-

cling series, the weight average of the molecular weight,Mw, of PP-2 had decreased somewhat (Table 2). Thepolydispersity (Mw/Mn) had decreased as well and theelongation-at-break values had started to drop andthereafter recover, which indicates that some parts ofthe samples had started to degrade. This indicates thatmolecular chain breakage has taken place in the highmolecular weight fraction. Long molecules are knownto be important for high elongation-at-break values[26,27]. Similar results, i.e. decreased Mw and Mw/Mn,have also been reported from studies of repeated extru-sion of commercial polypropylene grades [9,28,29].

4. Conclusions

� During simulated recycling of PP-1, a zig-zagshaped elongation-at-break curve was observed.The elongation-at-break dropped upon eachageing step and increased again upon eachextrusion step. We suggest three explanations tothis behaviour: (1) changes in degree of crystal-linity, (2) surface degradation, and (3) dilution ofdegraded polymer chains after re-extrusion. Theelongation behaviour is probably explained by acombination of all three mechanisms.

� In conclusion, simulated recycling degraded thepolypropylene materials faster than repeatedextrusion or ageing, and better mimics realityand should therefore be a valuable tool forrecycling investigations.

� Combining extrusion and ageing definitely affectsthe degradation behaviour of the material. Sodoes the order in which extrusion and acceleratedageing are performed. Simulated recycling orrepeated extrusion followed by accelerated age-ing more severely affects the material degradationthan ageing followed by repeated extrusion.

� Mechanical testing and elongation-at-breakshould be used with care, since the results maygive different indications of the status of thematerial, depending on the sample preparation.Re-extrusion most often tend to improve themechanical properties of a material, as was thecase for the PP-1 material.

� Mechanical recycling with the ambition to pro-duce high quality products is not an easy task.This investigation shows some of the difficulties,for example variation between different batchesin terms of stabilisation, contamination etc.

Acknowledgements

Mistra (Foundation for Strategic EnvironmentalResearch), Swedish Environmental protection Agencyand Formas (The Swedish Research Council for Envir-onment, Agricultural Sciences and Spatial Planning) aregratefully acknowledged for financial support. Strand-plast is acknowledged for supplying the PP materials,Ulrika Carlander (SP) for TOF-MALDI experiments,Peter Sjovall (SP) for TOF-SIMS experiments andAnders Martensson at the Department of PolymerTechnology at Chalmers for SEC measurements. SofiaKallquist and Mariann Pettersson for their assistancewith laboratory work at SP. Dr Arne Holmstrom (SP)and Professor Thomas Hjertberg (Chalmers) isacknowledged for reviewing and discussing the resultspresented in this paper.

References

[1] APME Insight into consumption and recovery in Western Eur-

ope. Available at: http://www.apme.org; 2001.

[2] Olund G, Eriksson E. Resthanteringsalternativ for plastfor-

packningar-en miljopaverkansbedomning. Goteborg: Chalmers

Industriteknik, Ekologik; 1998 [in Swedish].

[3] Boldizar A, Gevert T, Markinger M. Polym Recycling 1995;

1:249.

[4] Boldizar A, Jansson A, Gevert T, Moller K. Polym Degrad Stab

2000;68:317.

[5] Boldizar A, Gevert T, Markinger M. In: Polymer Processing

Society Asia/Australia meeting; 1996.

[6] Aurrekoetexa J, Sarrionandia MA, Urrutibeascoa I, Maspoch

ML. J Mater Sci 2001;36:2607.

[7] Fearon PK, Marshall N, Billingham NC, Bigger SW. J Appl

Polym Sci 2000;79:733.

Table 2

Molecular weight for PP-2 samples extruded once (step 1), and after

four extrusions and three ageing steps (step 7)

Material

Mn�10�3 Mw�10�3 Mw/Mn

PP-2 step 1

35 140 4.0

PP-2 step 7

34 129 3.9

The values are means of two measurements.

A. Jansson et al. / Polymer Degradation and Stability 82 (2003) 37–46 45

[8] Incarnato L, Scarfato P, Acierno D. Polym Eng Sci 1999;

39(4):749.

[9] Incarnato L, Scarfato P, Gorrasi G, Vittoria V, Acierno D.

Polym Eng Sci 1999;39(9):1661.

[10] Kartalis CN, Papaspyrides CD, Pfaendner R, Hoffmann K,

Herbst H. J Appl Polym Sci 1999;73:1775.

[11] Tzankova DN, Jilov N, La Mantia FP. Polym Degrad Stab 1997;

57:191.

[12] Dixon RK. IEEE Transaction on Elec Insulators 1980;El-15 No

4:331–40.

[13] Mori S, Barth HG. Size exclusion chromatography. Berlin:

Springer Verlag; 1999.

[14] Gensler R, Plummer CJG, Kausch H-H, Kramer E, Pauquet J-R,

Zweifwel H. Polym Degrad Stab 2000;67:195.

[15] Moller K, Jansson A, Sjovall P. Polym Degrad Stab 2003;80:345.

[16] Zweifel H. Stabilization of polymeric materials. 1st ed. vol. 1.

Springer Verlag; 1998.

[17] Camacho W, Karlsson S. Polym Degrad Stab 2002;78:385.

[18] Celina M, George GA. Polym Degrad Stab 1993;40:323.

[19] Celina M, George GA. Polym Degrad Stab 1995;50:89.

[20] Fayolle B, Audouin L, George GA, Verdu J. Polm Degrad Stab

2002;77:515.

[21] Gugumus F. Polym Degrad Stab 1996;52:145.

[22] Gugumus F. Polym Degrad Stab 1996;52:159.

[23] Knight JB, Calvert PD, Billingham NC. Polymer 1985;26:1713.

[24] Richters P. Macromolecules 1969;3(2):262.

[25] Ahlblad G, Gijsman P, Terselius B, Jansson A, Moller K. Polym

Degrad Stab 2001;73:15.

[26] Ibhadon AO. J Appl Polym Sci 1998;69:2657.

[27] Karger-Kocsis J. Microstructural aspects of fracture in poly-

propylene and in its filled, chopped fiber and fiber mat rein-

forced composites. In: Karger-Kocsis J, editor. Polypropylene

structure, blends and composites. Chapman & Hall; 1995.

p. 142–201.

[28] Gonzalez-Gonzalez VA, Neira-Velazquez G, Angulo-Sanchez JL.

Polym Degrad Stab 1998;60:33.

[29] Hoffmann K, Herbst H, Pfaendner R. Tailor-made stabilizer

systems boost recycled plastics in AddCon, Brussels, 1996.

46 A. Jansson et al. / Polymer Degradation and Stability 82 (2003) 37–46