The effect of extensive mechanical recycling on the properties of low density polyethylene

Polymer Degradation and Stability 97 (2012) 2262e2272

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The effect of extensive mechanical recycling on the properties of low density polyethylene Huiying Jin, Joamin Gonzalez-Gutierrez, Pavel Oblak, Barbara Zupan
ci
c*, Igor Emri Centre for Experimental Mechanics, Faculty of Mechanical Engineering, University of Ljubljana and Institute for Sustainable Innovative Technology, Pot za Brdom 104, 1125 Ljubljana, Slovenia

a r t i c l e i n f o

a b s t r a c t

Article history: Received 25 May 2012 Received in revised form 12 July 2012 Accepted 26 July 2012 Available online 14 August 2012

Low density polyethylene (LDPE) was exposed to one hundred (100) consecutive extensive extrusion cycles to simulate mechanical recycling. Collected samples were characterized by means of small amplitude oscillatory measurements to investigate rheological properties, by gel permeation chromatography (GPC) to measure molecular weight, and with differential scanning calorimetry (DSC) to study thermal properties. Finally, solid time-dependent mechanical properties were characterized by measuring creep compliance. The results show that simulated recycling did not significantly change the melting and crystallization temperatures of LDPE. However, results from rheological measurement, crystallinity, creep measurements and GPC suggest that thermal degradation and gelation of LDPE occur after extensive extrusion which leads to simultaneous chain scission and crosslinking of the polymer chains. It can be concluded that processability, measured by rheological parameters at high frequency and durability of LDPE measured by creep compliance, are only affected after the 40th extrusion cycle. These observations correspond to the molecular changes of LDPE measured through GPC, MFI and crystallinity calculations obtained from DSC measurements. Ó 2012 Elsevier Ltd. All rights reserved.

Keywords: Low density polyethylene Mechanical recycling Extrusion Differential scanning calorimetry Rheology Gel permeation chromatography

1. Introduction Plastics are ubiquitous in our everyday life; they are used in an enormous and expanding range of products as diverse as household appliances, packaging, construction, medicine, electronics, automotive and aerospace components. They have already displaced many traditional materials, such as wood, stone, leather, paper, metal, glass, and ceramic, in many applications. In 2010, total plastic production in Europe was 57 million tons, and post-consumer waste was 24.7 million tons of which 10.4 million tons (42%) were disposed of in landfills [1]. To reduce this large amount of plastic waste going into landfills, as well as, to conserve non-renewable fossil fuels used in plastic production, a strategy for sustainable development should include plastic recovery. There are mainly three options for plastics recovery: mechanical recycling, feedstock recycling and energy recovery [2]. Mechanical recycling is also known as physical recycling because plastics are ground down and then reprocessed into re-usable material through a physical rather than a chemical process. Feedstock recycling

* Corresponding author. Tel.: þ386 1 6207 103; fax: þ386 1 6207 110. E-mail address: [email protected] (B. Zupan
ci
c). 0141-3910/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.polymdegradstab.2012.07.039

which is also called chemical recycling consists of turning polymer chain into shorter hydrocarbon components that can be used as source for rebuilding new polymers. Energy recovery refers to incineration of waste materials to recover inherent energy. Mechanical recycling is the most widely practiced of these methods as it is relatively easy and economic. However, properties of mechanically recycled polymers do not remain the same because of degradation from heat, mechanical stress, oxidation and ultraviolet radiation during reprocessing and lifetime [3]. The nature of polymer and processing conditions determine the degree and type of polymer degradation, but all changes to which chemical structure was subjected are followed by the changes in the properties of the material. Polymers with moderate molecular weight under fairly mild processing conditions go through generally modest degradation. However, when applying more severe processing conditions, or using polymers with high molecular weight, and repeated processing operations can lead to a significant decrease in polymer characteristics [3]. Degradation of polyolefins during processing has been studied by several authors. In the literature, it is illustrated that during processing, polymeric materials are exposed to thermo-mechanical and thermo-oxidative degradation which are responsible for chain scission, branching and crosslinking reactions. The commonly accepted degradation mechanism of low density polyethylene

H. Jin et al. / Polymer Degradation and Stability 97 (2012) 2262e2272

(LDPE) is the simultaneous occurrence of both chain scission and molecular enlargement, although it shows a higher tendency to crosslink or chain branching whereas chain scission is overall dominating for more linear polymers such as linear low density polyethylene (LLDPE), high density polyethylene (HDPE) and polypropylene (PP) [4e16]. Kabdi et al. [9] found that mechanically recycled LDPE samples presented lower values of melt flow index (MFI) in comparison with virgin LDPE and they explained that this behaviour is because LDPE scraps were subjected to thermo- and photo-oxidative degradation during the processing or use, which leads to some crosslinking. Holmström et al. [11] stated that both chain scission and molecular enlargement occur simultaneously during thermal degradation of LDPE in nitrogen atmosphere having less than 1.16% oxygen. These two degradation mechanisms have opposite effects on the molecular weight and molecular weight distribution (MWD) of the polymer. The molecular weight decreases with increasing chain scission reaction whereas the molecular enlargement reaction accounts for shifting the MWD towards the high molecular weight end. La Mantia [3] reported, however, that in most processing operations, the variations of molecular weight caused by thermo-mechanical stress are very small, that is why only the Newtonian viscosity or the viscosity at low shear rates is strongly influenced by processing conditions. Several research groups have investigated the effects of mechanical recycling of a variety of polyolefins [7e19], but the effect of extensive mechanical recycling has not been reported. The current study investigates the effects of extensive mechanical recycling on several physical properties of low density polyethylene (LDPE). The obtained results can be used for defining or optimizing LDPE processing conditions for different product applications, as well as for predicting long-term (time-dependent) behaviour of material. 2. Materials and methods 2.1. Materials Low density polyethylene (LDPE) OKITENÒ 245S produced by DIOKI (Croatia) with a density of 0.924 g/cm3 and melting point of 114  C was used for the purposes of this investigation. Suggested uses of OKITENÒ 245S include manufacturing of thin slippery films, films and containers for food packing and co-extrusion with other

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polyolefins. The manufacturer indicated the presence of a slip agent and a thermal stabilizer in the material, but no antiblocking agent. However, no further details were provided. 2.2. Repeated extrusion process and sample preparation Mechanical recycling was simulated by means of repeated extrusion. A Haake Polylab PTW 16/40 OS twin screw extruder produced by Thermo Scientific (Germany) was used to perform one hundred extrusion cycles. Fig. 1 shows the arrangement of the corotating extrusion screws used for the process of simulated recycling. Detailed explanation of the configuration can be found in [20]. LDPE was extruded at screw rotation of 150 min1, a processing temperature of 240  C and with a throughput between 1200 and 1300 g/h. For the first ten extrusions 45 cylindrical samples (with diameter d ¼ 6 mm, and length l ¼ 200 mm) were collected from each cycle and thereafter the same amount of samples from every next tenth extrusion cycle. The remaining extruded material was cooled in a water tank and then pelletized using a Thermo Haake pelletizer (type 557-2685) before being submitted to a new extrusion cycle (Fig. 2). 2.3. Sample characterization 2.3.1. Viscosity measurements Rheological properties of each sample were measured in Haake MARS (Modular Advanced Rheometer System), Thermo Scientific (Germany). All frequency sweep tests were performed at 240  C, using plateeplate geometry (D ¼ 20 mm, gap ¼ 0.5 mm), at frequency increasing from 0.1 Hz (0.628 rad/s) to 100 Hz (628.32 rad/s). All measurements were performed applying a shear stress of 300 Pa, previously determined to be within the linear viscoelastic domain of the material. For each recycled material ten repetitions were performed. In this study, viscosity results are presented as the magnitude of the complex viscosity (jh*j), which is calculated as shown in equation (1),

sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi  0 2  00 2 G G þ jh*j ¼

u

u

(1)

where G0 is the shear storage modulus, G00 is the shear loss modulus and u is the angular frequency.

Fig. 1. Arrangements of the co-rotating screws.

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Fig. 2. Simulated mechanical recycling method.

2.3.2. Melt flow index (MFI) measurements Melt flow index (MFI) measurements were performed according to standards ASTM D1238 and ISO 1133. For this purpose, an Extrusion Plastometer of the Italian producer ATS Faar S.p.a., was used. The melt flow properties were measured at the temperature of 190  C by applying pressure via prescribed gravimetric weight of 2.16 kg. MFI measurements were performed for the virgin LDPE and recycled materials from selected consecutive extrusions (1st, 10th, 40th, 60th and 100th) with two repetitions. 2.3.3. Differential scanning calorimetry Differential scanning calorimetry (DSC) analysis was carried out according to ISO 11357. All the measurements were performed on DSC7 instrument, Perkin Elmer, USA, in a nitrogen atmosphere. Mass of used specimens was 5.5  0.1 mg. Heating and cooling rate was 20  C/min. Average values of six replicates have been used. Melting temperature Tm and crystallization temperature Tc as well as heat of fusion Hf were evaluated from a second heating run. The degree of crystallinity (Xcr) was estimated using equation (2), DH100% crystalline ¼ 293 J/g [18],

Xcr ¼

DHf DH100%

(2)

2.3.4. Creep measurements Creep measurements were performed in the Shear Creep Torsiometer which has been developed in the Centre for Experimental Mechanics of the Faculty of Mechanical Engineering, University of Ljubljana, and is presented in detail elsewhere [21e23]. Fig. 3 schematically presents this measuring system consisting of three parts: (i) mechanical device enabling torsional loading of a test specimen in solid state, (ii) heating chamber, and (iii) measurements and regulation setup including a computer and software, used for data acquiring and analysis. All moving parts have been equipped with aerostatic bearings to minimize friction in the measuring device. The smallest torque loading is 1  105 Nm, and the smallest measurable twist is 1.0  104 rad, the twist measuring range is from 0 to 2.8 or 0.05 rad. The test specimen is placed in the heating chamber built for temperatures from room temperature to 160  C, which can maintain the temperature at 0.5  C during the time interval of 104 s. For the purpose of shear creep measurements LDPE cylindrical sample of 5.70  0.05 mm in diameter and 40.0  1.0 mm in length, was fixed by an adhesive to metal holders, specifically adapted for

Fig. 3. Scheme of shear creep torsiometer.

proper gripping in the torsiometer. In the first phase of shear creep measurements each LDPE sample was heated up from room temperature to 90  C over a period of 3 h, kept at this temperature for 4 h, and then gradually cooled to 30  C over a period of 8 h. After this phase, the measurement was continued with loading, unloading and conditioning of the sample at various temperatures (30, 50, 60, 70 and 80  C). Loading phase lasted 3 h at a constant temperature, then the loading torque was removed and the temperature was increased to the next measuring temperature. The process of heating, and then maintaining the specimen at this elevated temperature took about 3 h. The specimen was again torque-loaded, and the procedure of exchanging the phases of loading, unloading and conditioning was repeated over the selected

Fig. 4. Schematic representation of the temperature-loading profile of the shear creep measurements.

H. Jin et al. / Polymer Degradation and Stability 97 (2012) 2262e2272

JðtÞ ¼

gðtÞ r,4ðtÞ ¼ s0 l,s0

(3)

where s0 is the applied stress, r denotes radius of the circular cross section of the sample, l length of the sample, and g(t) timedependent shear strain response. Applied torsional load resulted in shear strain not exceeding 0.04%, regarding each measured segment of deformational response at different temperatures following the profile displayed in Fig. 4. 2.3.5. Gel permeation chromatography The molecular weight analysis of the samples was carried out by high temperature gel permeation chromatography (HT GPC) at Polymer Standards Service GmbH (PSS, Mainz, Germany). The columns used were PSS Polefin columns, with a column length of 300 mm and a particle size of 10 microns and with solvent 1,2,4trichlorobenzen. The injection system used was PolymerChar GPC-IR with injection volume of 200 ml, using a flow rate of 1.0 ml/ min, and the temperature was set at 150  C. The results are presented here as the number-average molecular weight, Mn, weightaverage molecular weight, Mw, and as polydispersity index (PDI). 3. Results and discussion 3.1. Rheological and MFI measurements Simulated recycling of LDPE was carried out through melt extrusion at high temperature (240  C) and at high extrusion speed (150 min1) under atmospheric condition. All materials tested show a decreasing value of viscosity as angular frequency increases (i.e. shear thinning) in the frequency range studied (0.628e261.6 rad/s) and selected temperature of 240  C (Fig. 5). The effect of simulated

recycling on the viscosity of LDPE at 240  C is shown in the flow curves shown in Figs. 5 and 6. In Fig. 5, it can be clearly seen that the complex viscosity (equation (2)) at low frequencies (jh*j at 0.628 rad/ s) drastically increases (approximately ten times) with extrusion cycles. The complex viscosity (equation (2)) at high frequencies (jh*j at 291.6 rad/s) is, on the contrary, less affected by the processing. This is in agreement with information provided by La Mantia [3]. The typical shear rates used in industrial processing operations is between 100 and 1000 s1, which through the CoxeMerz rule [24] corresponds to 100e1000 rad/s. Within the high frequency range a change by a factor of about 1.5 in the viscosity was observed as a result of 100 extrusions. On the other hand, the complex viscosity at low frequencies was found to differ by about one decade (Fig. 5). This means, that during usual processing operations, mechanical recycling would cause negligible variations of the viscosity and of the extrusion pressure or torque at the selected processing temperature (240  C). At high frequency complex viscosity slightly decreases after first extrusion then gradually increases from second extrusion until the 100th extrusion (Fig. 6). Due to the inherent characteristics of polymeric materials, particularly polyethylene, they are unfavourably affected by processing parameters such as temperature, shear and the presence of oxygen. These three factors can lead to thermomechanical and thermo-oxidative degradation as discussed in the Introduction. Thermal degradation of polymers mainly results in chain scission and/or crosslinking reactions competitively and simultaneously. In the case of LDPE, both chain scission and crosslinking reactions can occur during the simulated recycling process. However, as shown in the literature LDPE has a higher tendency to crosslink [4e16], and therefore the increase of viscosity

a 16000 -> min

Complex Viscosity (Pa.s)

measuring temperature range. Fig. 4 schematically presents the temperature-loading profile of the performed creep experiments. Time needed for each measurement was around 42 h. For each recycled material creep measurements were performed on three different specimens (i.e. three independent repetitions) according to the temperature profile of one measuring cycle shown in Fig. 4. The stresses applied to each specimen were selected as to remain in the linear viscoelastic region for all materials and ranged from s0 ¼ 6.7  104 Pa at 30  C to 2.7  104 at 80  C. Preliminary measurements made it evident that the loadings selected do not affect the linearity of the LDPE viscoelastic behaviour. The response of the material was measured in terms of rotational angle of the specimen, 4(t) as a function of time and then recalculated to the material shear creep compliance:

T = 240 °C

12000

8000

4000

0 0

b

100000

100 Virgin LDPE Extrusion 30 Extrusion 60 Extrusion 100

5

10

20

30

40

50

60

70

80

90 100

70

80

90 100

max

T = 240 °C

150

100

50

0

10 0.1

2

200 max->

Complex Viscosity (Pa.s)

Complex Viscosity (Pa.s)

1000

1

Number of Extrusion Cycles

T = 240 °C

10000

2265

1

10 100 Angular Frequency (rad/s)

1000

Fig. 5. Complex viscosity as function of angular frequency for LDPE of different extrusion cycles.

0

1

2

5

10

20

30

40

50

60

Number of Extrusion Cycles Fig. 6. Variation of complex viscosity in dependence of the number of extrusion cycles at: (a) minimum (0.628 rad/s), and (b) maximum (291.6 rad/s) frequency.

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a

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10000 -> min T = 240 °C

G' (Pa)

8000

6000

4000

2000

0 0

1

2

5

10

20

30

40

50

60

70

80

90 100

Number of Extrusion Cycles

b

40000 max ->

max

T = 240 °C

G' (Pa)

32000

24000

16000

8000

0 0

1

2

5

10

20

30

40

50

60

70

80

90 100

Number of Extrusion Cycles Fig. 7. Variation of storage modulus G0 in dependence of the number of extrusion cycles at: (a) minimum (0.628 rad/s), and (b) maximum (291.6 rad/s) frequency.

with extrusion cycles (Fig. 6) can be related to a reduction in polymeric chain mobility due to crosslinking. Storage modulus, G0 , and loss modulus, G00 , displayed in Figs. 7 and 8, follow a very similar trend as shown by the complex viscosity. Viscoelastic moduli increase with the number of extrusion cycles, which is indicative of a compact and entangled microstructural network [25]. The overall trend for the measured rheological moduli (G0 and G00 ) as a function of extrusion cycles might be explained as follows: due to mechanical and thermal degradation, chain scission and crosslinking happens simultaneously, however, during the first extrusion chain scission is slightly more dominant than crosslinking, while from the second extrusion crosslinking prevails over chain scission. The smaller polymeric chains produced after chain scission, have more mobility under shear flow, while crosslinked chains have reduced mobility under the same conditions. These results are in accordance with the results presented by other authors [8,9,13,14,17,18]. Fig. 9 shows storage modulus and loss modulus for extrusion cycles 0, 40, and 100 as function of angular frequency at 240  C. Virgin LDPE (Fig. 9a) showed a predominantly viscous response (G00 > G0 ) at up to a frequency of 64 rad/s when both viscoelastic moduli have the same value (G00 ¼ G0 ); as the frequency increased beyond 64 rad/s virgin LDPE behaved more elastic than viscous (G00 < G0 ). It was observed that after 30 extrusion cycles, a predominant elastic behaviour (G00 < G0 ) is shown in the entire frequency interval tested and there was no crossover point between the two viscoelastic moduli (Fig. 9b), being almost parallel to each other in the whole measurement range; this is a typical behaviour of a gel structure [26]. As the number of extrusions

Fig. 8. Variation of loss modulus G00 in dependence of the number of extrusion cycles at: (a) minimum (0.628 rad/s), and (b) maximum (291.6 rad/s) frequency.

continued up to 100, the distance between values of G00 and G0 steadily increased (Fig. 9c) indicating a stronger solid-like behaviour for the extensively extruded LDPE at all the studied frequencies. The point when the storage modulus is equal to the loss modulus can be considered a good estimate of the gel point [32]. At the gel point polymers start to behave like a network with restricted polymer chain mobility thus preventing easy flow. Fig. 10 displays the evolution of the crossing point between G0 and G00 as function of angular frequency. The decrease of the angular frequency at which G0 and G00 intersects indicates that the relaxation time of polymeric chains is increasing with extrusion cycles, which additionally supports the idea that crosslinking has occurred during the extrusion process. It is important to point out that starting at the 30th extrusion cycle viscoelastic moduli were not intercepting within the measuring window; this is the reason for presenting interception angular frequency only up to the 20th extrusion cycle (Fig. 10). Fig. 11 presents the results of the melt flow index (MFI) measurements. It is evident that the MFI decreases with extrusion cycles, and there is practically no throughput when it comes to the MFI of 100-times extruded material. Decrease in MFI with consecutive extrusions indicates a decrease in the ability of molten LDPE to flow under pressure. This suggests reduction in the mobility of polymeric chains, which can be attributed to crosslinking or molecular enlargement, since both mechanisms decrease chain mobility. However, results presented below indicate that crosslinking is probably a dominating mechanism. MFI measurements are in agreement with the rheological data presented previously.

H. Jin et al. / Polymer Degradation and Stability 97 (2012) 2262e2272

a

2267

100000

G' & G'' (Pa.s)

T = 240 °C

10000

1000 Virgin LDPE

G' G''

100 0.1

b

1

10 100 Angular Frequency (rad/s)

1000 Fig. 10. Variation of angular frequency at which G0 and G00 intersect as function of the number of extrusion cycles.

100000

G' & G'' (Pa.s)

T = 240 °C

aliphatic vinyl); or as a result of increasing crosslinking which can reduce the close packing ability of the polymer chains, and hence decrease the polymer crystallinity [5,10]. Therefore, a decrease in crystallinity with increasing number of extrusion cycles is another result that supports the hypothesis that crosslinking prevails over chain scission during mechanical recycling.

10000

1000 Extrusion 40

3.3. Effect of recycling on time-dependent properties

G' G''

100 0.1

c

1

10 100 Angular Frequency (rad/s)

1000

100000

G' & G'' (Pa.s)

T = 240 °C

10000

1000 Extrusion 100 G' G''

100 0.1

1

10 100 Angular Frequency (rad/s)

1000

Fig. 9. G0 and G00 of LDPE after (a) 0, (b) 40, and (c) 100 Extrusion cycles.

3.3.1. Shear creep compliance and temperature dependence Determining retained properties and durability is among the most important tasks when evaluating the possibility of mechanical recycling of plastic waste. As previously described in the research methodology, creep measurements were performed at five different temperatures: 30, 50, 60, 70, and 80  C, approximately 3 h (slightly longer than 104 s) for each of them. The data obtained for virgin LDPE, LDPE exposed to 40 and 100 extrusion cycles is given as an example to illustrate how the creep data is processed in order to familiarize the reader with this kind of analysis (Fig. 14aec). To study the effect of mechanical recycling on temperature stability of polymeric products, we generated isochrones of the creep compliance in dependence of the temperatures within the measuring temperature range (30e80  C) at the reference time of 1000 and 10,000 s (approximately 15 min and 3 h, respectively). As an example, temperature-dependent values of the creep compliance for virgin LDPE at the two selected reference times are indicated with the dots and arrows in Fig. 14a, and are analysed in continuation.

3.2. Differential scanning calorimetry Melting temperature and crystallization temperature of all samples before and after recycling were measured. As can be seen in Fig. 12, there is no significant difference in values measured between virgin LDPE and recycled ones. The maximum differences in melting temperature and crystallization temperature between recycled and virgin LDPE are 2.93% and 1.17% respectively, which are within experimental error. The percentage of crystallinity (Xcr) of recycled LDPE is found to be more or less constant up to the 40th extrusion cycle, and then starts to decrease somewhere between the 40th and 50th extrusions as presented in Fig. 13. According to the literature, the decrease in crystallinity could be attributed to the creation of structural irregularity by formation of short branches in the backbone chain and groups (e.g. hydroxide, hydroperoxide, carbonyl,

Fig. 11. Melt flow index (MFI) in g/10 min for virgin LDPE, LDPE exposed to 1, 10, 40, 60 and 100 extrusions.

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Fig. 12. Melting temperature and crystallization temperature in dependence of the number of extrusion cycles.

construct the creep compliance master curves of virgin LDPE (Fig. 17). Timeetemperature shifting of the response function segments was performed by using the closed form shifting (CFS) methodology [27]. Since no measurements were performed exactly at 40  C, the shift factors for creating the master curve were modelled by using Arrhenius equation to generate the master curve at the reference temperature of 40  C. Several authors [28e30] reported that far from the glass transition temperature (more than 100  C above Tg) which is the case of investigated polyethylene, the temperature shift factors follow an Arrhenius relationship. From Arrhenius-type modelling of the temperature shift factors one can determine parameter H, i.e. the activation energy for material flow regarding the investigated temperature range. Fig. 16 shows the diagram of calculated activation energies in dependence of the number of extrusion cycles by applying Arrhenius model to obtained temperature shift factors.

As indicated in Fig. 14a we extracted values of creep compliance from the segments, measured at different temperatures, and analysed temperature dependence via generated isochrones within measuring temperature range. Since experimental window of each measured segment is limited, the reference time can be selected only up to 10,000 s. Isochrones of the creep compliance are presented in Fig. 15a and b at the reference times of 1000 and 10,000 s for the case of virgin LDPE, and LDPE exposed to 10, 40, 60, and 100 extrusions. From Fig. 15a and b it can be seen that for the investigated materials as the temperature increases there is also an increase in the creep compliance at any of the selected times, which is the expected behaviour for thermoplastic polymers. Complete analysis of the temperature dependence including all investigated materials exposed to different number of extrusions has shown that the temperature stability worsens after the 60th extrusion cycle up to approximately 25%, this can be seen in Fig. 15a and b in which the creep compliances of the material extruded 60 times shows the highest increase in shear creep compliance as a function of temperature. However as extrusion cycles increase there is an improvement in the temperature stability; as observed in Fig. 15a and b the curve for the material extruded 100 times shows smaller values of shear creep compliances than material extruded 60 times and the values even come close to the values of virgin LDPE. 3.3.2. Shear creep compliance and time dependence For the purpose of the time-dependent creep properties a reference temperature of 40  C was selected and the timee temperature superposition principle was applied, in order to

Fig. 13. Variation of crystallinity in dependence of the number of extrusion cycles.

Fig. 14. Creep compliance as a function of time for (a) virgin LDPE, (b) LDPE exposed to 40 extrusions, and (c) LDPE exposed to 100 extrusions measured at T ¼ 30, 50, 60, 70, and 80  C.

H. Jin et al. / Polymer Degradation and Stability 97 (2012) 2262e2272

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a

b

Fig. 17. Creep compliance master curve as a function of time for virgin LDPE, LDPE exposed to 40 and 100 extrusions at the reference temperature, Tref ¼ 40  C.

Differences in obtained activation energies relatively to obtained activation energy for the virgin LDPE are only up to 11.5%. Nevertheless, decrease of activation energy that can be observed up to the 5th extrusion cycle in respect to the virgin LDPE, may indicate presence of chain scission process and higher mobility of polymeric chains, while increase of activation energy in the 80the 100th back to the level of activation energy for the virgin LDPE might be related to the crosslinking process, and decreased ability for re-arrangement of the molecules.

To study the effect of mechanical recycling on long-time stability of polymeric products, the isochronal values of the creep compliance after 3 years (t z 107.98 s) and 10 years (t z 108.50 s) were chosen from master curves of recycled materials, shifted according to Arrhenius equation to the reference temperature of 40  C. The two isochronal values of the creep compliance are indicated in Fig. 17, which shows time-dependent shear creep compliance of virgin and recycled materials at reference temperature of 40  C. Comparison of creep compliance values after 3 and 10 year are presented in Fig. 18 for virgin and recycled materials at reference temperature of 40  C. From Fig. 18, it can be seen that the maximum differences in creep compliance between virgin LDPE and recycled ones is less than 10% until the 40th extrusion cycle after 3, as well as after 10 years. However, after 40th extrusion cycle, there was a higher increase of creep compliance which gave maximum differences of approximately up to 25% and 26% after 3 years and 10 years, respectively. These results are in accordance with the observed change of crystallinity in DSC measurements (Fig. 13). At the molecular level, crystalline regions provide resistance for polymer chains rearrangement [31], therefore it is reasonable to expect that magnitude of creep would be inversely proportional to the degree of crystallinity. In addition, the increase/decrease of creep compliance in comparison to the creep compliance of virgin LDPE reflects the changes in mobility of molecular chains. In this respect it relatively well corresponds to the indicated decrease/increase in activation energy (Fig. 16).

Fig. 16. Activation energy in temperature range 30e80  C as function of the number of extrusion cycles.

Fig. 18. Creep compliance as function of the number of extrusion cycles after 3 and 10 years.

Fig. 15. Creep compliance as a function of temperature for virgin LDPE, and LDPE exposed to 10, 40, 60, and 100 extrusions at the reference times (a) tref ¼ 1000 s, and (b) tref ¼ 10,000 s.

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a

Fig. 19. Proportions of the insoluble part of the material for virgin and extruded LDPE in 1,2,4-trichlorobenzen solution.

b

3.4. Gel permeation chromatography Gel permeation chromatography was performed on virgin and extruded LDPE. Prepared solutions of extruded materials in 1,2,4-trichlorobenzen for GPC measurements revealed very different proportions of insoluble parts of the material depending on the number of recycling (extrusions) (Fig. 19). Solubility results show that proportion of insoluble fraction of the material increases significantly after five extrusions, and reaches a maximum of 37% after 100 extrusions. Increase of the insoluble part is additional proof of the occurrence of crosslinking during the extensive extrusion of LDPE. In continuation, three normalized molecular weight distribution curves are shown in Fig. 20 as an example of GPC results. It can be seen that as the number of extrusions cycles increase there is a shift in the peak of the molecular distribution to smaller values. The shape of the distribution curve also changes as number of extrusions progress, for example after 40 extrusions the curve widens and a tail at high molecular weight is visible, but after 100 extrusions the curve becomes narrower and tails disappears. These changes in the molecular weight distribution suggest that there are processes that reduce the molecular weight such as chain scission; however at the same time there are processes of molecular enlargement and crosslinking. A better way to summarize GPC results is to look at the average molecular number (Mn), average molecular weight (Mw) and the polydispersity index (PDI). The number-average molecular weight, Mn and the weightaverage molecular weight, Mw of all samples before and after recycling are shown in Fig. 21. In addition, polydispersity index

Fig. 20. Normalized molecular weight distribution for virgin LDPE, extruded 40 and 100 times.

Fig. 21. (a) Mn, and (b) Mw as function of number of extrusion cycles.

(PDI) (Mw/Mn) was calculated from above two values and its evolution with extrusion cycles is shown in Fig. 22. Results on molecular weight distribution (MWD) obtained from GPC can be used to investigate degradation of polymers. If chain scission occurs, the molecular weight distribution drifts toward smaller values and the progressive changes in molecular weight distribution can be modelled if it can be assumed that scission occurs randomly. If crosslinking occurs, the molecular weight distribution drifts in the opposite direction. If both scission and crosslinking occur, then the shape of the molecular weight distribution changes considerably [31] as shown in Fig. 20. GPC results indicate that thermal-mechanical and oxidative degradation during mechanical recycling results in a general shift of the molecular weight distribution (MWD) toward lower

Fig. 22. PDI as function of number of extrusion cycles.

H. Jin et al. / Polymer Degradation and Stability 97 (2012) 2262e2272

molecular weights. As is shown in Fig. 21a, Mn decreases with increasing extrusion cycles which suggests an increase in the amount of low molecular weight material. This is an indication of chain scission happening during extensive extrusion cycles. However, molecular enlargement can also occur. This is evidenced by an intermediate increase of Mw (Fig. 21b) between the 2nd and 10th, and 20th and 40th recycling cycle and similarly for PDI between the 20th and 40th recycling cycle (Fig. 22), which indicates the development of high molecular weight tail of the MWD. After the 50th extrusion cycle all the MWD parameters decay, such changes in MWD may be linked to a more extensive degradation that also manifested as a decrease in crystallinity and an increase in creep compliance after the 50th extrusion cycle. Therefore, these results are additional evidence that chain scission and crosslinking happened simultaneously during mechanical recycling.

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evidenced by an increase of Mw and PDI which indicates the development of a high molecular weight tail of the MWD. Therefore, these results are additional evidence that chain scission, molecular enlargement and crosslinking happened simultaneously during mechanical recycling. Finally, solubility result of extruded LDPE in 1,2,4-trichlorobenzen shows that extensive extrusion increases the amount of insoluble part of the material, which brings additional support to the reasoning that crosslinking occurs during the repeated extrusions and is more dominant than the chain scission as the number of extrusions increases. With these results on hand, it could be concluded that LDPE could be extruded for up to 40 times without significantly changing its processability and long-time mechanical properties. This could be attributed to the two competing structural rearrangement mechanisms that simultaneously occur: chain scission and crosslinking.

4. Conclusions In the present work, the effect of extensive simulated mechanical recycling of LDPE by means of melt extrusion up to one hundred (100) cycles on rheological, thermal and time-dependent mechanical properties as well as molecular weight distribution were evaluated. It is necessary to emphasize that conclusions based on the obtained results are strictly valid for the temperature conditions used in the experiments, and should be used with care for describing behaviour of recycled materials in other conditions. It was observed that rheological functions (G0 , G00 and jh*j) change after each extrusion cycle. After one extrusion cycle rheological values dropped, but in the subsequent extrusion cycles there was an increasing general trend. These changes indicate that mechanical recycling leads to molecular rearrangements caused by thermo-mechanical and thermo-oxidative degradation. Rheological measurements indicate that chain scission and crosslinking are happening simultaneously in LDPE. However, it seems that at a particular extrusion one mechanism dominates over the other; for example during the first extrusion chain scission is slightly more dominant than crosslinking, whereas at higher number of extrusions the crosslinking prevails. MFI results are in agreement with rheological results, and show that melt flow index decreases with the number of extrusion cycles. DSC measurement showed that there is no significant difference in melting temperature and crystallization temperature measured between virgin LDPE and recycled ones. However, a decrease in crystallinity starting at the 50th extrusion was observed which can be attributed to the creation of structural irregularities as a result of increasing crosslinking which can reduce the close packing ability of the polymer chains, and hence, decreases the polymer crystallinity. Therefore, decrease in crystallinity is another result that supports the prevalence of crosslinking over chain scission during extensive mechanical recycling. From creep measurements, it can be seen that mechanical recycling does not affect time-dependent mechanical properties until the 40th extrusion cycle, since no significant change was observed in the creep compliance at 3 and 10 years. However, after the 50th extrusion cycle, there was a more pronounced increase of creep compliance when analysing time as well as temperature stability. This result is in accordance with the change of crystallinity measured by DSC. Since, at the molecular level, crystalline regions provide the stiff resistance for polymer chains to creep, therefore it is reasonable to believe that the lower the crystallinity, the more a solid polymer can creep. GPC results show that Mn decreases with increasing extrusion cycles which is an indication of chain scission happening during repeated extrusion cycles. However, GPC measurements also indicate that molecular enlargement may have occurred. This is

Acknowledgements This work was conducted with the financial support by the European Commission through the Erasmus Mundus Executive Agency and the Slovenian Research Agency e ARRS. We would like to acknowledge the ISOKON d.o.o. (Slovenia) for performing melt flow index measurements. Grateful acknowledgement goes also to Samo Kozoderc and Matej Gabr
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