Polymer Degradation and Stability 91 (2006) 1226e1232www.elsevier.com/locate/polydegstab

Degradation of pre-aged polymers exposed to simulated recycling:Properties and thermal stability

Sharbel Luzuriaga a, Jana Kovarova b,*, Ivan Fortelny b

a Department of Macromolecular Chemistry, Faculty of Natural Sciences, Masaryk University, Kotlarska 2, CZ 611 37 Brno, Czech Republicb Department of Polymer Materials, Institute of Macromolecular Chemistry, Academy of Sciences of the Czech Republic,

Heyrovskeho nam. 2, CZ-162 06 Praha 6, Czech Republic

Received 22 June 2005; received in revised form 14 September 2005; accepted 15 September 2005

Available online 2 November 2005

Abstract

Accelerated thermal and photo-aging of four homopolymers, low-density polyethylene (LDPE), high-density polyethylene (HDPE), polypro-pylene (PP) and high-impact polystyrene (HIPS), was performed and the impact of subsequent reprocessing conditions on their properties stud-ied. Polymer samples oven-aged at 100 �C for varying periods of time or UV irradiated in a Weather-o-meter (WOM) at l Z 340 nm werereprocessed in a Brabender plasticorder at 190 �C/60 rpm for 10 min. Chemical changes and the evolution of rheological and mechanical prop-erties accompanying the gradual degradation of the individual polymers were monitored and evaluated (DSC, FTIR, colorimetric method, MFI,tensile impact strength). LDPE and HIPS were found to be more susceptible to thermo-oxidation than HDPE and PP, whereas HDPE and PPwere affected to a greater extent by UV exposure; the crucial role here is being played by the stabilization of the studied resins. In HDPE thescission and crosslinking reactions competed both in thermo-and photo-degradation. In the case of LDPE, scission prevailed over branchingduring thermo-oxidation, whereas photo-oxidation of the same sample led predominantly to crosslinking. Abrupt deterioration of the LDPErheological properties after one week of thermal exposure was suppressed by re-stabilization. The scission reaction was also predominant forPP during thermo-oxidation, and it took place even faster during UV exposure. In the case of HIPS a slight photo-degradation of PS matrixis accompanied by simultaneous crosslinking of the polybutadiene component.� 2005 Elsevier Ltd. All rights reserved.

Keywords: Accelerated weathering; Simulated recycling; Polyolefins; Styrenics; Reprocessing; Re-stabilization

1. Introduction

Plastics present in the municipal waste stream e polyolefins,styrene plastics, poly(vinyl chloride) (PVC), poly(ethyleneterephthalate) (PET), etc. e show a wide range of materialdeterioration as a consequence of degradation reactions due tothe high processing temperature and then to the effect ofoutdoor exposure in the course of the first service life-time[1]. Because the use of PVC for packaging is restricted, andPET bottles are separated from municipal plastic waste, themost important components of mixed post-consumer plasticdesignated for material recycling in the Czech Republic are

* Corresponding author. Tel.: C420296809210; fax: C420296809410.

E-mail address: kovarova@imc.cas.cz (J. Kovarova).

0141-3910/$ - see front matter � 2005 Elsevier Ltd. All rights reserved.

doi:10.1016/j.polymdegradstab.2005.09.004

various grades of polyethylene, isotactic polypropylene, andstyrenic plastics [2].

Oxidative degradation as a complex of radical chain reac-tion leads, in the polymers under study, mainly to chain scis-sion and/or crosslinking, giving rise to a series of chemicaltransformations in their structures [3e5]. PE shows a highertendency to crosslinking [6e8] whereas PP and HIPS areprone to chain scission [8,9]. Chain-scission reactions alsocontribute to the formation of vinylidene and vinyl groupsand other unsaturation on the polyethylene chains [7].

Since oxygen is ubiquitous, auto-oxidation is the predomi-nant reaction during the processing and aging of polymers.FTIR analysis of aged polymers has detected a wide rangeof oxygen-containing groups such as aldehydes, ethers,ketones, peroxides, carboxylic acids, etc., resulting from the

1227S. Luzuriaga et al. / Polymer Degradation and Stability 91 (2006) 1226e1232

auto-oxidation of polymers [7e10]. Consequently, post-consumer materials are more susceptible to further thermaldegradation during subsequent mechanical recycling due tothe effect of accumulated oxygen-containing groups in theirstructure. They participate in further reactions altering themolecular structure, and hence morphology, and the mechanicalproperties of the polymers [11,12].

Nowadays, studies on the recycling possibilities of post-consumer plastic waste are being carried out [13e17]. Eachof the materials is subjected to repeated cycles of extrusion,alternating with aging or being performed separately, in orderto predict the durability of a given material for a second use onthe basis of mechanical and chemical criteria [18e20].

Results arising from those studies do not always give a clearnotion of the complex processes taking place during process-ing and aging. Recent work [18] on simulated recycling ofpre-aged PP showed that the order in which extrusion and/oraging is performed had an influence on the final mechanicalproperties. It was demonstrated that, in a series of samplessubjected to an alternating aging/re-extrusion sequence, theimpact strength of aged samples showed a tendency to recoverfrom a drop caused by a previous aging step. It was also con-cluded that the material deteriorated more rapidly than in thecase when samples were subjected only to extrusion or agingseparately. Moreover, it was observed that reprocessing afteraging caused the material to deteriorate to a greater extentthan in the case where the samples were first processed andthen subjected to aging [17e19].

In terms of recyclability, HDPE seems to be very suitablefor recycling under the conditions of a proper re-stabilization.It was shown that HDPE scraps did not degrade to any sig-nificant degree during 10 cycles of simulated recycling[14,16,21]. Similar satisfactory results were achieved by Kar-talis during the recycling of post-used PP [6,16,17]. However,this approach could be hardly achieved in the absence of sta-bilizers, as reported by Cruz and Zanin [22]; a slight decay inMFI values was observed during the reprocessing of unstabi-lised HDPE. In the case of LDPE, the combination of reproc-essing and accelerated thermal aging degrades the materialfaster than either processing or aging performed separately[15]. The LDPE virgin material, used in short-life products’applications usually does not contain any stabilizer and recy-cling of post-used LDPE can be hardly achieved withoutre-stabilization.

Concerning the material rheology, the flow properties arerather influenced by the effect of repeated extrusions, in whichthe simultaneous effects of high temperature and mechanicalstress are involved. Owing to the processing conditions, thepolymer chains are extended in a shear field in the directionof the motion, and it is assumed that the highest molecularweight chains are preferentially broken during the scission, re-sulting in the changes of molecular-weight distribution, whichare directly connected to polymer viscosity [23,26], whileshorter molecules participate in branching reactions. On theother hand, thermal and photo-aging are related to a surfacedegradation process, which displays a spatial gradient charac-ter [24,25,30]. The course of degradation is a function of the

oxygen diffusion coefficient and morphology of the polymers,and it is manifested mainly in the loss of the material mechan-ical properties [17,18].

Natural and accelerated degradations of HIPS were shownto proceed by two-phase oxidation, where air oxygen preferen-tially attacks the polybutadiene component [28], the oxidationrate being a function of the polybutadiene content. Changes inHIPS tensile properties under high-temperature reprocessingconditions are attributed to the cleavage of bonds betweenthe matrix and the rubbery modifier [9,28].

In spite of papers on the aging of many of the common pol-ymers, and also on their reprocessing, there still exist inconsis-tencies with regard to their relative stabilities; these areprobably caused by significant structural differences amongthe respective polymer grades. They may also be due to varia-tions in the protective stabilizers applied. Moreover, the agingand reprocessing of polymers under study are rarely encoun-tered in combination.

In this work we have focused our attention on both the ef-fect of accelerated aging and the subsequent reprocessing onsome selected useful properties of common grades of polyole-fins and styrenics, which form a significant fraction of theCzech domestic waste stream. Knowledge arising from thiswork will be utilized in further research dealing with the useof aged components in the mechanical recycling of compatibi-lised polymer blends.

2. Materials and methods

� LDPE: Low-density polyethylene Bralen RA-2-19(Slovnaft Bratislava, Slovakia), density 918 kg/m3,Mw Z 120 000 (unstabilised).� PP: Isotactic polypropylene Mosten 52522 (Chemopetrol

Litvınov, Czech Republic).� HDPE: High-density polyethylene Liten BB 29 (Chemo-

petrol Litvınov, Czech Republic), Mw Z 420 000.� HIPS: High-impact polystyrene Krasten 562E (Kau�cuk

Co., Kralupy n/V, Czech Republic), Mw Z 190 000, 7%polybutadiene.� Secondary amine stabilizer (Dus): Trade name Dusantox L,

Duslo Salla, Slovakia: mixture of 60 wt.% of N-(1,3-di-methylbutyl)-N#-phenyl-1,4-phenylenediamine and 40 wt.%of N-[4-(a,a#-dimethylbenzyl)phenyl]-N#-(1,3-dimethyl-butyl)-1,4-phenylenediamine.� Commercial phenolic antioxidant: Irganox 1010

(Irg.1010), Ciba Specialty Chemicals.

2.1. Aging and sample preparation

2.1.1. Oven-agingPolymer granules were oven-aged at 100 �C for various pe-

riods of time. Aged granules were kneaded in the W50EHTchamber of a Brabender plasticorder at 190 �C for 10 minand the hot molten bulk was pressed at 200 �C for 4 min ina Fontijne table press, and then cooled in a cold press. Tensilestrength, crystallinity, rheology and thermo-oxidative stabilitywere monitored on specimens formed from these plates.

1228 S. Luzuriaga et al. / Polymer Degradation and Stability 91 (2006) 1226e1232

2.1.2. Photo-oxidationThin polymer plates (150 ! 150 ! 2 mm) were prepared

by compression moulding after kneading in a Brabender plas-ticorder at 190 �C/60 rpm for 6 min. They were then exposedto UV irradiation in a Xenon Weather-o-meter device atl Z 340 nm for one or two weeks. In the following step, theirradiated plates were reprocessed (Brabender plasticorder at190 �C/60 rpm for 10 min), and re-formed into new compres-sion-moulded specimens. Re-formed samples were again eval-uated in terms of rheological, thermal and tensile properties.

2.2. Sample characterization

2.2.1. Hydroperoxide contentHydroperoxide content was obtained by means of spectro-

colorimetric measurement of aged samples [29]. This analyti-cal method is based on the stoichiometric oxidation of Fe2C

ions by the hydroperoxides present leading to Fe3C ions.They react with thiocyanate, giving rise to an intense colourof the solution, with the measured intensity being proportionalto the hydroperoxide content.

2.2.2. Crystallinity, melting enthalpy, and oxidative stabilityDSC measurements were performed in nitrogen at a heating

rate 10 �C/min on a PerkineElmer DSC-7 apparatus, equippedwith Pyris 1 software, and calibrated by an indium standard.Melting temperature Tm and heat of fusion Hf were evaluatedfrom the first and second heating runs for each sample, and thecrystallinity content was calculated. Oxidation exotherms wereobtained in air at a heating rate 3 �C/min in a temperaturerange 130e300 �C, on about 5 mg of the sample.

2.2.3. MFIMelt flow index (MFI) of the polymers was monitored ac-

cording to ISO 1133 at a temperature of 190 �C, and load10 kg (method F ); in case of PP method D at 190 �C andload 2.16 kg was selected. The MFI corresponds to the massof polymer that passes through a standard capillary, in aninterval of 10 min, at a given applied pressure (load).

2.2.4. Tensile impact strengthTensile impact strength, a3, was measured at room temper-

ature using a Zwick tester, which was equipped with a specialfixture for the test specimens according to DIN 53 448. Plateswere cut into dog-bone-shape specimens with the short narrowsection from the plates 1.6 mm thick, and the width of the nar-row section 3 mm. The maximum pendulum energy was 2 J.The values obtained are presented as arithmetic means of themeasurements on 10 specimens.

3. Results and discussion

3.1. Thermo-oxidation e oven-aging experiment

Hydroperoxide concentration was used as one of the indica-tors of the extent of polymer degradation in relation to

exposure time in the oven-aging experiments performed at100 �C. Exposure times shown in Table 1 correspond to thetime period at which most significant structural changes ofthe studied materials were detected.

LDPE has poor stabilization, if any, since it is mostlyintended for short-life application products. This fact wasevidenced by the enhanced sensitivity of oven-aged LDPEgranules to degradation. LDPE granules were the first showingconsiderable symptoms of degradation; yellowing and hydro-peroxide formation were detected during the early stage of ex-posure (Fig. 1). Structural factors, such as morphology, alsocontributed to the degradation profile. It is known that oxida-tion of polyolefins is mostly restricted to the amorphous phasebecause oxygen diffusion across the compact crystalline do-mains is almost impossible. Thus, highly amorphous LDPEsuffers from oxidation reactions to a greater extent than themore crystalline HDPE. After two weeks of LDPE exposure,the hydroperoxide concentration curve showed a plateau(Fig. 1), indicating a steady-state hydroperoxide concentra-tion. Similar behaviour was observed also with HIPS samples.In contrast, HDPE and PP remained very stable; even after 10months of oven exposure no hydroperoxide formation couldbe detected in these materials. In both polymers, the stabiliza-tion system comprising a combination of sterically hindered

Table 1

Changes in polymer characteristics with increasing oven-aging exposure time

at 100 �C

Material Exposure

time

(days)

Impact

strength

a3 (kJ/m2)

Crystallinity

(%)

Tonset

( �C)

MFI

(g/10 min)

Granules

LDPE 0 e 32.7 191 3.8

7 e 33.0 190 a

HDPE 0 e 53.2 237 2.8

311 e 67.8 229 2.9

PP 0 e 52.2 212 2.2b

334 e 55.2 212 2.2b

HIPS 0 e e 181 9.4

20 e e 179 8.2

93 e e 175 8.9

159 e e 181 9.3

PlatesLDPE 0 127 29.0 190 3.2

LDPE C 0.5%

Dus

7 76 34.4 192 e

LDPE C 0.5%

Dus C 0.5%

Irg1010

7 62 31.7 208 e

HDPE 0 129 52.9 232 2.9

311 98 61.2 219 3.4

PP 0 51 49.6 210 2.5b

334 49 45.4 207 2.7

HIPS 0 26 e 178 9.4

20 39 e 176 10.7

93 36 e 167 8.4

159 28 e 169 9.9

a It was not possible to measure the MFI of the samples due to the significant

material degradation.b 190 �C/2.16 kg.

1229S. Luzuriaga et al. / Polymer Degradation and Stability 91 (2006) 1226e1232

phenol and organic phosphite provided the studied materialwith long-term resistance against oven-aging at 100 �C. It isobvious that the presence of stabilizers affects the degradationbehaviour of polymers substantially, and must be consideredin the interpretation of experimental results [27].

During thermal aging of both PEs, crosslinking and chain-scission reactions occur simultaneously in a competitive way.The crystallinity increase observed in oven-aged LDPE platesand HDPE indicates that scission prevailed over crosslinking.The crystallinity increase probably also contributed to the dropof the original impact strength (IS) observed in LDPE andHDPE plates prepared from oven-aged granules (Table 1).Nevertheless, the crystallinity of the oven-aged HDPE plateis lower than in the respective granules, indicating a negativeeffect of processing conditions. PP samples seemed to be verystable under thermal aging, only a slight decrease in IS beingobserved, while MFI and the oxidative onset temperature(Tonset) remained almost constant. On the other hand, theLDPE granules subjected to thermal aging at 100 �C for 7days were not further processible without additional stabiliza-tion. The stabilizer used was sometimes a phenylenediamineprocessing stabilizer, Dusantox L, but even more satisfying re-sults in terms of oxidative stability were achieved by the appli-cation of a mixture of 0.5% Dusantox L and 0.5% Irganox1010 (Table 1).

Aging of HIPS, as known from the literature, proceeds asa two-phase oxidation, in which the rate depends on the con-tent of the polybutadiene (PB) component [28]. The yellowingobserved in our exposed granules is caused by the presence ofchromophores, and their interaction with the aromatic ring ofthe PS matrix.

From low-temperature DSC measurements, it was foundthat degradation in the HIPS system occurs at different levels.In the early stage of aging, the PS matrix remained almost in-tact, as can be observed from the MFI values in Table 1, where-as crosslinking reactions take place in the PB phase, as couldbe detected by a slight shift of its Tg to higher temperatures(Table 2). The polybutadiene phase, as the more sensitive com-ponent towards oxidation, due to the unsaturated double bondsin the structure, first undergoes degradation which is associatedwith an increase in tensile strength in aged specimens observedafter the second week of oven exposure. This could result partlyfrom crosslinked PB inclusions and tentatively also from their

00,5

11,5

22,5

33,5

44,5

5

0 5 10 15 20 25 30 35Exposure time (days)

RO

OH

[m

mo

l/l]

Fig. 1. Hydroperoxide formation in LDPE granules at 100 �C during oven-

aging.

additional grafting to the PS matrix via hydroperoxides formedby oxidation reactions on PB.

3.2. Photo-oxidation: exposure in Weather-o-meter(WOM) at l Z 340 nm

3.2.1. Formation of oxygenated groupsHydroperoxide group formation and its decomposition in

the course of photo-aging or subsequent reprocessing, is animportant aspect, which may justify, among others, the fasterdegradation of materials subjected to simulated recycling[20,32].

Table 3 shows the hydroperoxide evolution in photoagedand reprocessed samples, in which the basic stabilization didnot comprise light stabilizers. UV light exposure leads to in-creased hydroperoxide formation in PP and HIPS samples.Since WOM exposure of PE favours crosslinking over scission[24,25] and since in contrast to PP, polyethylene hydroperox-ides readily photolyze on UV exposure, the detected

Table 2

Polybutadiene glass-transition temperature (Tg) in HIPS oven-aged for different

exposure time

Oven-aging exposure time (days) PB Tg ( �C)

HIPS (0) �97.5

HIPS (93) �95.8

HIPS (159) �91.1

Table 3

Hydroperoxide concentration and oxidation stability changes with increasing

time of accelerated photo-aging of polymers in WOM

Exposure time (days) ROOH c (mmol/l) Oxidation onset Tonset ( �C)

LDPE

0 0.01 190

6 0.06 185

6(RE)a 0.05 191

14 0.05 189

14(RE) 0.05 185

HDPE

0 e 232

6 e 197

6(RE)a e 194

14 e 195

14(RE)a e 192

PP

0 0.03 210

6 0.06 186

6(RE)a 0.06 191

14 0.48 187

14(RE)a 0.09 183

HIPS

0 3.41 171

6 3.54 170

6(RE)a 1.78 166

14 3.46 168

14(RE)a 1.83 166

a After re-extrusion.

1230 S. Luzuriaga et al. / Polymer Degradation and Stability 91 (2006) 1226e1232

hydroperoxide concentration is lower in LDPE and is notdetectable in HDPE photoaged samples (Table 3). On theother hand, the higher hydroperoxide concentration in PPwas expected due to structural factors which favour scissionover crosslinking [23]. The surprisingly high values ofROOH concentration found in the original virgin sample ofHIPS, prior to aging, probably originate from the technologi-cal manufacturing process, where organic peroxides serve asreactive initiating species in grafting PS on PB particles.

The concentration of hydroperoxides generated decreases sig-nificantly in the course of reprocessing (190 �C) and subsequentmoulding (200 �C), obviously due to their decomposition underhigh-temperature processing conditions, as can be seen fromthe values corresponding to reprocessed samples in Table 3.

3.2.2. Onset temperatureThe shift of the onset temperature Tonset (Table 3) to lower

values indicates the deterioration of the oxidative stability ofaged polymers. In PP and HIPS this shift can be associatedmainly with the presence and formation of oxygenated groupsand the consequent increased stabilizer consumption; in thecase of HDPE this decrease is rather related to the depletionof the stabilizer, because no formation of ROOH was detectedin the HDPE samples after photo-aging. Moreover, stericallyhindered phenolic antioxidants, applied as basic stabilizers in

polyolefins and HIPS, are not active as light stabilizers, becausethey decompose and deactivate under the influence of UV light.

Certain irregularities observed in the measured onset tem-peratures (and shown in Table 3) can be attributed to theheterogeneous character of photo-oxidation, where photo-degradation is restricted mainly to the surface of the sampleand to differences in the migration of antioxidant to the sur-face during subsequent thermo-processing of the samples[24]. On the other hand, the relatively smaller changes inoxidation onset temperature in unstabilised LDPE are associatedwith structural changes of the unprotected aged samples.

3.2.3. Crystallinity variationThe reorganization [18] of smaller chains resulting from PP

scission into more compact structures contributes to the

Table 4

Crystallinity variation in polymer samples exposed to WOM aging (UV) for

various periods of time and after reprocessing (RE)

Exposure time

(days)

PP crystallinity

(%)

HDPE crystallinity

(%)

LDPE crystallinity

(%)

0 53.0 58.2 34.3

6UV 53.7 53.8 32.8

6UV(RE) 47.2 58.1 35.2

14UV 57.2 60.1 24.2

14UV(RE) 44.4 63.3 33.2

0

15

30

45

60

75

90

105

120

135

Im

pact stren

gth

(kJ/m

2)

0 14 RE6 REExposure time (days)

LDPE

0

20

40

60

80

100

120

140

0 6 RE 14 14 REExposure time (days)

HDPE(a) (b)

0

15

30

45

60

Im

pac

t stren

gth

(kJ/m

2)

0 6 RE 14 14 REExposure time (days)

PP

0

5

10

15

20

25

30

Im

pact stren

gth

(kJ/m

2)

Im

pact stren

gth

(kJ/m

2)

0 6 RE 14 14 REExposure time (days)

HIPS(c) (d)

6 14 6

6 6

Fig. 2. Changes in tensile impact strength of plates exposed for different periods of time in WOM and after reprocessing (RE): (a) LDPE, (b) HDPE, (c) PP, and

(d) HIPS.

1231S. Luzuriaga et al. / Polymer Degradation and Stability 91 (2006) 1226e1232

increase in the crystallinity of PP specimens aged in WOM for14 days, as shown in Table 4. The contribution of other phe-nomena, such as the annealing effect and lamellar thickening,can be also taken into consideration [8]. The presence of chem-ical irregularities formed by hydroperoxide decomposition dur-ing reprocessing of PP probably makes the re-crystallizationprocess [19,30] more difficult, and consequently leads tothe decrease of crystallinity of the reprocessed PP samples(see Table 4). On the other hand, HDPE samples showincreased crystallinity rather in the later stages of photo-aging,and even after reprocessing, probably due to the increased roleof scission reactions. With respect to the observed crystallinityvariation, similar random behaviour was also noted byCarrasco et al. [31] in studies carried out on HDPE aging byUV irradiation.

3.2.4. Mechanical and rheological propertiesAs can be seen in Fig. 2, UV irradiation has a negative ef-

fect on the tensile impact strength of WOM-exposed LDPEand HDPE. The deterioration is caused by the influenceof two factors: surface defects and changes in the polymermolecular weight (crosslinking/chain scission). During thefirst week of irradiation, the surface degradation counterbalan-ces the positive effect of the molecular-weight increase (due toLDPE and HDPE crosslinking) on the tensile impact proper-ties, the final result being an overall decrease in the tensileimpact strength, as seen in Fig. 2a and b, respectively.

During the further prolonged irradiation time, the contribu-tion of molecular weight seems to be crucial in the interpreta-tion of the impact strength changes of LDPE and HDPE. Thisbehaviour is demonstrated by the difference in the course ofphoto-oxidation of LDPE and HDPE samples. The observedincrease of crystallinity (Table 4) indicates that HDPE under-goes scission from the second week of irradiation on, unlikethe more amorphous LDPE, which displays signs of crosslink-ing occurring predominantly in the amorphous region [33].Consequently, the impact strength of HDPE decreases duringthe second week due to the cooperative action of surface deg-radation and molecular-weight decrease, whereas the LDPEimpact strength displays a slight improvement due to

LDPE

02468

1012141618

0 6 RE Exposure time (days)

MF

I (g

/10 m

in

)

6 14 14 RE

Fig. 3. MFI of photoaged and consecutively reprocessed LDPE samples.

increasing molecular weight. For PP samples (Fig. 2c) the im-pact strength remained almost unaltered.

The tensile impact strength of irradiated reprocessed sam-ples shows a zigzag course, especially in HIPS (Fig. 2d). Itcan be attributed to surface degradation during photo-agingand to the subsequent impact-strength recovery after thesecond processing, resulting from the remixing of the degradedsurface into the bulk, as was observed in previous work [18].

3.2.5. MFICrosslinking reactions prevail in HDPE and LDPE, which

are shown by the lower melt flow indices after 6 days of pho-to-aging in WOM (Figs. 3 and 4). On the other hand, duringthe consecutive reprocessing of pre-aged HDPE and LDPEsamples, decomposition of hydroperoxides, chain scissions,and other chemical irregularities appear, as manifestedin higher values of MFI. The hydroperoxide decompositionin the course of the second processing also significantlyenhanced the rate of chain cleavage in PP, as observed inFig. 5. Concerning HIPS, a slight increase in MFI values wasdisplayed after the first week of exposure, and these valuesremained almost unchanged for the rest of the exposure time,even in reprocessed samples (Fig. 6).

HDPE

00.5

11.5

22.5

33.5

44.5

5

MF

I (g

/10 m

in

)

0 6 RE Exposure time (days)

6 14 14 RE

Fig. 4. MFI of photoaged and consecutively reprocessed HDPE samples.

PP

0

5

10

15

20

25

0 6 6 RE 14 14 REExposure time (days)

MF

I (g

/10 m

in

)

Fig. 5. MFI of photoaged and consecutively reprocessed PP samples (method

D at 190 �C/2.16 kg load).

1232 S. Luzuriaga et al. / Polymer Degradation and Stability 91 (2006) 1226e1232

4. Conclusions

4.1. Thermo-oxidation

Basic stabilization applied in the studied polyolefins andHIPS has different efficiencies, depending on the aging condi-tions. Because it was unprotected by stabilizers, LDPE wasrather sensitive towards degradation during oven-aging. It wasshown that LDPE, oven-aged for 7 days at 100 �C could notbe used for reprocessing because of too low a viscosity; a suit-able combination of diamine and phenol stabilizers suppresseddegradation during reprocessing. Modified polystyrene showedtwo-phase oxidation, where oxidation of polybutadiene compo-nent took place initially, leading to crosslinking, whereas thepolystyrene matrix remained almost intact during a longerperiod of time. On the other hand, HDPE and PP were wellstabilized against thermo-oxidation, retaining acceptablemechanical properties and thermo-oxidative stability even after10 months of oven-aging, hence qualifying such materials assuitable for recycling purposes.

4.2. Photo-oxidation

During accelerated photo-aging in the Weather-o-meter(WOM) device, PP was most prone to deterioration, due tostructural factors, and a lack of stabilization against light.Photo-aging of PP and HIPS in the WOM led to chain scis-sion. In the case of HDPE or LDPE, a crosslinking reactionprevailed in the early stage of irradiation; however, withincreasing time of exposure, the more crystalline HDPEunderwent scission, whereas LDPE still displayed signs ofcrosslinking. The formation of hydroperoxides during theaging phase, and their further decay, was shown to play a signi-ficant role in the faster chain scission in materials subjected tosimulated recycling, showing a pronounced increase in meltflow indices after reprocessing in most cases.

In general, it can be said that structural inhomogeneities builtup in thermo- and photoaged samples have an accelerating roleduring subsequent reprocessing and it is therefore mandatory toapply a proper re-stabilization package prior to reprocessing.

HIPS

0

2

4

6

8

10

12

14

0 6 6 RE 14 14 REExposure time (days)

MF

I (g

/10 m

in

)

Fig. 6. MFI of photoaged and consecutively reprocessed HIPS samples.

It should also be mentioned that the present results and con-clusions are valid for the grades of polymers applied in thiswork, and hence their transfer to other grades of these poly-mers should be made with caution.

Acknowledgement

Financial support from the Academy of Sciences ofthe Czech Republic, AVOZ 405 005 05, is gratefullyacknowledged.

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