The thermo-oxidative degradation of metallocene polyethylenes: Part 2: Thermal oxidation in the melt state

Polymer Degradation and Stability 91 (2006) 1363e1372 www.elsevier.com/locate/polydegstab

The thermo-oxidative degradation of metallocene polyethylenes: Part 2: Thermal oxidation in the melt state Eric M. Hoa`ng a, Norman S. Allen a,*, Christopher M. Liauw a, Eusebio Fonta´n b, Pilar Lafuente b a

Manchester Metropolitan University, Department of Chemistry and Materials, Center for Materials Science, Chester Street, Manchester M1 5GD, UK b REPSOL-YPF, Carretera Nacional V, km 18, 28931 Mostoles, Madrid, Spain Received 9 June 2005; accepted 26 July 2005 Available online 3 October 2005

Abstract The thermo-oxidative melt degradation of different metallocene polyethylenes (mPEs) was investigated in a torque rheometer open to air at 225
C and 10 rpm. The mPEs differed essentially according to their initial melt index, molar mass distribution, density and ash content, but one characteristic was changed at a time in order to assess the influence of each specific property in the thermo-oxidative degradation of the PEs investigated. Crosslinking was found to dominate at the early stages of degradation during mastication for most polymers where reactions of alkyl radicals to vinyl groups were considered to be the dominant reaction. Furthermore, discolouration was attributed to both excessive levels of catalyst residues and extensive formation of conjugated systems. Finally, it was concluded that the polymer melt viscosity, i.e., molar mass and shape of molar mass distribution, appeared to govern the processing stability of the mPE. These results confirm the importance of shear as the major source for initiation of free radicals formed by homolytic fission caused via mechanical cleavage of polymer chains. Ó 2005 Elsevier Ltd. All rights reserved. Keywords: Polyethylene; Metallocene; Degradation; Thermo-oxidation; Melt oxidation

1. Introduction PE degradation occurs most rapidly during melt processing, when polymer is exposed to severe conditions. During melt processing operations, PE is simultaneously subjected to high temperatures, high shearing forces and molecular oxygen. Important physical and chemical modification inevitably occurs at this stage and affects profoundly its subsequent service performance. A consequence of such thermo-oxidation during melt processing includes the formation of oxidation products. Thermo-oxidation may also affect molar mass changes with resultant melt viscosity changes. Viscosity increases or decreases depending on whether scission or crosslinking

* Corresponding author. Tel.: C44 161 247 1432; fax: C44 161 247 6357. E-mail address: [email protected] (N.S. Allen). 0141-3910/$ - see front matter Ó 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.polymdegradstab.2005.07.018

predominates. The nature of the physico-chemical processes occurring on processing PE has been recently re-examined [1e5]. This work was based on processing various PEs with different properties (polymerisation catalyst technology, vinyl content, crystallinity/density, etc.) in a mixer open to air in the temperature range of 150e200
C. The present study deals with the thermo-oxidative degradation of various metallocene polyethylenes (mPEs) in the melt state. The mPEs differed essentially according to their initial melt index, molar mass distribution, density and ash content, but one characteristic was changed at a time in order to assess the influence of each specific property in the thermo-oxidative degradation of the PEs investigated under dynamic conditions (high temperature and shear). The processing degradation study was carried out using a Brabender PlasticorderÒ mixer open to air to simulate aerobic processing conditions. This approach was used rather than a completely closed (anaerobic)

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mode in an attempt to simulate the worst possible processing conditions. Mixing torque was measured at 15, 30, 45, 60, 90 and 120 min at a rotor speed of 10 rpm and a temperature of 225
C. As the torque is a function of viscosity, torque profiles could estimate the extent of degradation of the masticated material. Melt flow rate (MFR) of degraded samples was also followed as a function of the residence time in the open mixer for characterising both the type of degradation (chain scission or crosslinking) and the extent of degradation of the polymers [6]. Vinyl and trans-vinylene unsaturation groups content were determined for all masticated samples. Finally, carbonyl growth and hydroperoxide stability were evaluated. 2. Experimental 2.1. Materials The polymers used in this work were non-stabilised polyethylenes produced via the metallocene catalyst technology, and were supplied by REPSOL-YPF (Madrid, Spain). Each PE was defined with distinct properties such as: -

Melt Index (190
C, 2.16 kg) Molar mass distribution (M w =M n ) Density Ash content

Polyethylenes properties are compiled in Table 1. As can be observed, mPE-REF was used as a reference material, and essentially one characteristic (melt index, molar mass distribution, density and ash content) was changed at a time in order to assess the influence of each specific property in the thermo-oxidative degradation of those PEs in the solid and the melt state. 2.2. Melt degradation experiment The degradation of pure polymers under processing conditions (high temperature and shear) was assessed using a torque rheometer. Torque rheometer such as Brabender PlasticorderÒ measures the torque required to masticate a standard sized plastic sample at a predetermined temperature. The torque is a function of viscosity, and it changes with time if degradation of the plastic occurs [7]. The torque rheometer can be used to determine the rheological profile of a plastic and the effect of stabilisers thereon.

PAD PAD PAD PAD PAD

296/4 248/12 201/2 281/1 295/1

Label mPE-REF mPE-MI mPE-MMD mPE-HD mPE-ASH

2.3. Melt flow rate The melt flow rate (MFR) test is a simple and convenient method for characterising both the type of degradation (chain scission or crosslinking) and the extent of degradation of a polymer since the MFI is inversely related to the molar mass of the polymer and is indicative of the flow characteristics of the molten polymer. Therefore, a decrease in the MFR of the polymer is indicative of crosslinking and is usually observed during melt processing of polymers such as Phillips PE. On the other hand, an increase in MFR relates to chain scission, which leads to a molar mass reduction, and this is usually the dominant mode of breakdown of PP in the melt. MFR was measured using a Ray-RanÒ Melt Flow Indexer capillary melt viscometer by applying a standard weight of 10 kg at a melt temperature of 190
C, in accordance with ASTM D1238. 2.4. Determination of unsaturation groups Unsaturated species, such as vinyl end-groups, have been found to play an active role in the degradation of PE. Such species can generally be resolved by infrared spectroscopy. Vinyl and trans-vinylene unsaturation groups in PE and their infrared properties [9] are summarized in Table 2. The number of unsaturated groups (CaC/1000C) was calculated according to the following equation [8]: CaC=1000CZ

MI (g/10 min)

M w =M n

1.1 8.0 1.2 1.0 1.1

2 2 9 2 2

Density (g/cm3)

Ash (%)

0.919 0.920 0.922 0.954 0.920

0.018 e 0.022 0.020 0.035

A 3d

where A is the absorbance, 3 is the extinction coefficient, and d is film thickness (mm). Table 2 Infrared properties of unsaturation groups in polyethylenes Unsaturated group Terminal vinyl

Table 1 Properties of the mPEs investigated Grade

For torque rheometer processing stability testing, 40 g of polymer sample was masticated in a Brabender PlasticorderÒ PL2000 torque rheometer (Duisburg, Germany) by using an open W 50 E mixer and cam blades. Samples were mixed for 15, 30, 45, 60, 90 and 120 min at a rotor speed of 10 rpm and a temperature of 225 G 5
C. The temperature was set to simulate high temperature processing. A low rotor speed was used, as low shear rate measurements are rheologically more sensitive to viscosity changes due to crosslinking [8].

Structure

Band position (cm1)

CH2

H C

C

H

H

CH2

trans-vinylene

H C

H

910

965

C CH2

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The level of vinyl unsaturation was determined by transmission FTIR spectroscopy of compression moulded films (200 mm thick) using a Nicolet NEXUS. FT-IR spectrometer, in the region 1050e800 cm1. Spectra were made from 32 scans with resolution set at 4 cm1.

a peroxide value which represents the sum of the concentrations of hydroperoxides, and several types of peroxides in each film sample.

2.5. Determination of carbonyl groups

3.1. Torqueetime profile

Rates of polymer oxidation were measured by monitoring the rate of formation of non-volatile carbonyl oxidation products. A Nicolet NEXUSÔ FT-IR spectrometer was used; spectrum was made up of 10 scans with resolution set at 4 cm1. The absorbance maximum between 1715 and 1720 cm1 was taken as a measure of the concentration of carbonyl compounds (mainly ketonic). Slight differences in film thickness were accounted for by dividing the absorbance by the thickness to give the following equation for carbonyl index (CI).

Torque profiles are presented in Figs. 1e5. According to Fig. 1, the torqueetime profile of the reference polymer mPE-REF could be divided into three main regions. The first region spans from 0 to ca. 72 min of mastication in the open mixer. The second region spans from ca. 72 to ca. 100 min. The third region spans the residence time range 100e 120 min. In the first two regions of the torqueetime profile of the reference polyethylene, an increase in torque during mastication was observed. This increase in torque, caused by an increase in the viscosity of the polymer melt, is a typical manifestation of crosslinking reactions that occur for PEs processed in a low oxygen atmosphere. The rate of change of mixing torque varied dramatically during the mixing process. The rates of change in mixing torque (slope) were calculated according to the following equation:

where A is the absorbance maximum at 1715e1720 cm1 and d is the film thickness (mm). 2.6. Determination of hydroperoxides

Torque2  Torque1 t2  t1

Results are presented in Table 3. The slope of torque increase in the interval [72e100] for mPE-REF was over four times greater than the slope in the first interval [0e72]. This reflects a greater extent in crosslinking reactions in the second part of the torque profile. After 100 min of processing in the open mixer, a significant decrease in torque was then displayed that suggests a tendency for chain scission reactions. These chain scission reactions are typical of polyethylenes processed in an excess of air. mPE-ASH, which contains a high initial ash content, did exhibit a similar torque profile compared to the reference

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Numerous methods are available for hydroperoxide detection. However, the iodometric method based on the oxidation of iodide ions is the most widely used of all colorimetric methods for hydroperoxide analysis. The hydroperoxide concentrations of oxidised film samples were determined using an iodine liberation procedure. This iodometric approach is based on the reduction of the peroxide bond by iodide [10]. Film fragments (0.5 g) were refluxed for 30 min with 10 cm3 of a mixture of glacial acetic acid (100%, GPRÔ BDH Chemicals Ltd) and propan-2-ol (99.8%, CHROMASOLVÒ Riedel-deHae¨n, HPLC grade) at a ratio by volume of (5:95, acetic acid:2-propanol) in the presence of 0.1 g of sodium iodide (99%, FISONS plc). Although a brief reflux (about 5 min) is normally required to give complete decomposition of hydroperoxides, a longer period was allowed since the production of I 3 from a film sample is limited by the slow diffusion of reagents into the polymer [11]. Furthermore, the use of a solvent mixture of acetic acid and isopropanol offers considerable advantages; while iodide is more active in acetic acid, the use of an alcohol as a solvent completely suppresses the interference caused by air oxidation of the iodide reagent [12]. As it has been stated that the presence of water in the acetic acid solvent retards the reaction between peroxide and iodide ions, the use of a 99% grade isopropanol is recommended [13]. At the end of the reflux period, the solution was cooled for 5 min, and the I 3 formed during the peroxide decomposition was determined using a Perkin Elmer Lambda 7 UV/Vis spectrophotometer at 420 nm in a 1-cm quartz cell. The calibration curve was obtained using cumene hydroperoxide (80%, BDH Chemicals Ltd). Because of the prolonged reflux conditions required, the method gives

SlopeZ

Torque [Nm]

A CIZ 100 d

3. Results and discussion

160 150 120

Time [min] Fig. 1. Torque profile for mPE-REF masticated in air for 120 min at 225
C and 10 rpm in a BrabenderÒ mixer.

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Torque [Nm]

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AR

0

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72

96

Temp [°C]

50

Temp [°C]

Torque [Nm]

1366

160

150 120

Time [min]

Fig. 2. Torque profile for mPE-ASH masticated in air for 120 min at 225
C and 10 rpm in a BrabenderÒ mixer.

Fig. 4. Torque profile for mPE-HD masticated in air for 120 min at 225
C and 10 rpm in a BrabenderÒ mixer.

material mPE-REF (Fig. 2). The three distinct parts were clearly distinguished with their specific slopes. The first part of the torque profile for mPE-ASH was very close to the one for mPE-REF with nearly the same slope value of C0.16. However, the degree of crosslinking during processing of mPE-ASH was lower than for the reference material. Indeed, a lower slope value of C0.34 for mPE-ASH was displayed compared to a value of C0.56 for mPE-REF. Furthermore, a higher extent of chain scission reactions predominated for mPE-ASH after about 100 min of processing in the open mixer, that was reflected by its lower slope value of 0.21 compared to a value of 0.10 for the reference polymer. In the case of mPE-MI (Fig. 3), a slight increase in torque was observed that indicates predominance toward crosslinking. Moreover, the very low torque change and the very low slope increase suggest a relatively high stability against degradation during mastication compared to the reference polymer mPE-REF.

Regarding the high-density polymer mPE-HD, crosslinking reactions were predominant according to the increase of torque during the processing degradation in the mixer as shown in Fig. 4. However, a higher rate of reaction was discerned from around 110 min of mastication as suggested by the greater slope value of C0.31 compared to a slope value of C0.10 for sample processed before 110 min. According to the slope values, the high-density polyethylene mPE-HD appeared to be more stable during thermo-oxidative processing in the open mixer than both the reference material mPE-REF and polyethylene mPE-ASH, which contains high ash residues. On the other hand, mPE-MMD, which exhibited an initially wide molar mass distribution, offers a different pattern compared to the other mPEs. Indeed, chain scission reactions appeared to predominate at the early stage of degradation during mastication in the open mixer (i.e. up to ca. 24 min), according to Fig. 5 and the negative slope (Table 3). After about 24 min of mastication in the open mixer, crosslinking reactions prevailed. However, the very low slope value of C0.03

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Torque [Nm]

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Temp [°C]

50

Temp [°C]

Torque [Nm]

Time [min]

160

150 120

Time [min]

Time [min]

Fig. 3. Torque profile for mPE-MI masticated in air for 120 min at 225
C and 10 rpm in a BrabenderÒ mixer.

Fig. 5. Torque profile for mPE-MMD masticated in air for 120 min at 225
C and 10 rpm in a BrabenderÒ mixer.

E.M. Hoa`ng et al. / Polymer Degradation and Stability 91 (2006) 1363e1372 Table 3 Slopes of torque evolution during mastication of mPE in an open mixer at 225
C and 10 rpm Mastication time intervals (min)

Slope (Nm/min)

mPE-REF

[0e72] [72e100] [100e120]

C0.16 C0.56 0.10

[0e72] [72e90] [90e120]

C0.17 C0.34 0.21

mPE-MI

[0e120]

C0.02

mPE-HD

[0e110] [110e120]

C0.10 C0.31

[0e24] [24e120]

0.04 C0.03

mPE-ASH

mPE-MMD

mPE-REF mPE-ASH mPE-MI mPE-HD mPE-MMD

90 80 70 60 50 40 30 20 10 0

suggests a relatively lower extent of crosslinking than mPEREF, mPE-ASH, and the high-density metallocene polyethylene mPE-HD. 3.2. Evolution of MFR during mastication The qualitative processing degradation of the investigated PEs provided by the torqueetime profiles was backed-up with complementary MFR data in order to characterise both the type of degradation (chain scission or crosslinking) and the extent of degradation of the polymers processed in the open mixer. MFR (190
C, 10 kg) results are compiled in Table 4. Changes in MFR, expressed in percentage of the initial MFR for processed materials, were evaluated as follows: %MFRZ

100

MFR Changes (%)

mPE

1367

MFRt 100 MFR0

where MFRt is the MFR value at mastication time t and MFR0 is the initial MFR value of the unaged material. Results are presented in Fig. 6. MFR data are in good agreement with the torque data. Similar initial reductions in MFR were observed for all masticated metallocene polyethylenes with the exception of mPE-MMD. This trend emphasises the predominance of crosslinking reactions over chain scission reactions during the mastication of the melts in the open mixer. In the case of mPE-MMD, the increased proportion of chain scission reactions suggested by the slight decrease in torque observed up to 24 min (Fig. 5) is confirmed by an increase in MFR exhibited between 15 and 30 min of mastication. However, the Table 4 MFR (190
C, 10 kg) data for masticated mPEs Mastication time (min)

mPE-REF (g/10 min)

mPE-MI (g/10 min)

mPE-MMD mPE-HD (g/10 min) (g/10 min)

mPE-ASH (g/10 min)

0 15 30 45 60 90

6.7 1.0 0.7 0.5 0.2 0.0

162.6 124.3 121.5 99.8 80.7 57.4

19.9 7.0 7.8 7.5 5.6 4.2

7.2 1.1 0.9 0.4 0.2 0.0

10.5 1.7 1.5 0.7 0.7 0.3

0

15

30

45

60

75

90

Mastication Time (min) Fig. 6. Decline in MFR (190
C, 10 kg) (expressed as percentage of the initial MFR) for mPEs masticated in air at 225
C and 10 rpm in a BrabenderÒ mixer.

sharp decline in MFR between 0 and 15 min of mastication indicates that crosslinking reactions predominated during the early stages of degradation, before chain scission reactions became more dominant between about 15 and 30 min. MFR data also confirm the greatest processing stability of mPE-MI relative to the other mPEs. The greatest retention of MFR was observed for this polymer; MFR was equal to nearly 40% of its initial value after 90 min of mastication. mPE-MMD also showed somewhat greater stability against processing degradation compared with the other mPEs, as predicted by its torque profile (Fig. 5). The observed competition between crosslinking and chain scission reactions that occur during processing of mPE-MMD may balance the melt viscosity of the processed material, hence may be responsible for its relatively superior melt viscosity stability. Its final MFR value at 90 min of processing was equal to about 20% of its initial value. Finally, very poor processing stability was recorded for mPE-REF, mPE-ASH and the high-density polyethylene mPE-HD. The latter samples yielded MFR values that were worth less than 20% of their initial value after only 15 min of mastication. The high-density polymer (mPE-HD), however, displayed slightly higher melt stability than mPE-REF and mPE-ASH as suggested previously by torque profiles. 3.3. Evolution of vinyl unsaturation concentration during mastication In order to correlate crosslinking reactions observed in the previous section with structural changes during processing, the concentrations of vinyl groups in processed mPEs in the open mixer were monitored by IR spectroscopy. Results are presented in Fig. 7. A decrease in vinyl unsaturation was observed for all mPEs masticated in air at 225
C and 10 rpm. The vinyl reductions observed during processing suggest that crosslinking reactions may be initiated at the vinyl unsaturation groups, and may also confirm the well-recognised addition reaction of alkyl radicals to vinyl unsaturation [8,9,14e17]. In order to compare the extent of vinyl decomposition between the different polymers

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mPE-MMD also exhibited a relatively low decay in vinyl during mastication, here the vinyl concentration did not fall lower than 80% of its initial value during the mastication period investigated. The relatively small reduction in vinyl unsaturation in mPE-MMD during mastication may be attributed to the competition between crosslinking and chain scission reactions as previously suggested by its torqueetime profile (Fig. 5) and MFR data (Fig. 6). For instance, b-scission of secondary radicals yields vinyl unsaturation [18,19] as shown in Reaction (1).

0.350

[Vinyl] (C=C/1000 C)

0.300 0.250 0.200 0.150 0.100 0.050

R 0.000 0

30

60

90

120

Fig. 7. Evolution of vinyl unsaturation groups of mPEs masticated in air at 225
C and 10 rpm in a BrabenderÒ mixer.

studied, the concentration of vinyl groups in processed materials was expressed as a percentage of the initial vinyl group content according to the following equation: ½vinylt 100 ½vinyl0

where [vinyl]0 is the initial vinyl concentration and [vinyl]t is the vinyl concentration at mastication time t. Results are presented in Fig. 8. It can be clearly seen that mPE-MI, which showed the lowest increase in melt viscosity (crosslinking) and the slowest reduction in MFR with increasing mastication time (Fig. 6), displayed the lowest level of vinyl decomposition during mastication. The vinyl unsaturation content of the latter sample was no less than 90% of its initial value over the mastication period investigated.

100 90

Vinyl Changes (%)

80 70 60 50 40

mPE-REF mPE-ASH mPE-MI mPE-HD mPE-MMD

30 20 10 0 0

CH2

R'

Secondary radical

R

CH CH2

+

R'

(1)

vinyl group

Mastication Time (min)

mPE-REF mPE-ASH mPE-MI mPE-HD mPE-MMD

Vinyl ð%ÞZ

CH

30

60

90

120

In the case of both mPE-REF and mPE-ASH, the decay in vinyl concentration was much more pronounced. In these samples, a considerable (40%) drop in vinyl unsaturation was recorded after 15 min of mastication; this is consistent with the previously observed decline in MFR of those samples. Furthermore, the decrease in vinyl concentration seemed to stabilise after the initial drop for both polymers and reached a limiting value after approximately 90 min of mastication. This is also in good concordance with the MFR data that showed little reduction after mastication periods longer than 60 min (Fig. 6). In the case of the high-density polymer mPE-HD, no dramatic decline in vinyl content during the early stages of mastication was observed; this is in direct contrast to mPE-REF and mPE-ASH. The observed trend in vinyl unsaturation for mPE-HD was quite surprising since this polymer displayed a similar drop in MFR to mPE-ASH and mPE-REF. This difference in vinyl decomposition profile may suggest other competitive reactions during mastication that may also involve the vinyl unsaturation; the next section is devoted to this aspect. 3.4. Evolution of trans-vinylene unsaturation concentration during mastication Concentrations of trans-vinylene groups in mPEs as a function of mastication time were determined by IR spectroscopy. Results are presented in Fig. 9. It is evident from Fig. 9 that all mPEs showed a slight increase in the level of trans-vinylene unsaturation at short mastication times. In the cases of all samples, apart from mPE-REF and mPE-ASH, this small initial increase was to a limiting value; however, with the latter two samples, a significant increase in trans-vinylene group concentration was recorded at longer mastication times. As for vinyl unsaturation, the concentrations of trans-vinylene groups in the masticated samples were expressed as a percentage of the initial trans-vinylene group concentration as follows:

Mastication Time (min) Fig. 8. Changes of vinyl unsaturation groups, expressed in percentage of initial vinyl unsaturation content, in mPEs masticated in air at 225
C and 10 rpm in a BrabenderÒ mixer.



 ½trans  vinylenet %Z 100  100 ½trans  vinylene0

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0.16

t-vinylene changes (%)

0.14

[t-vinylene] (C=C/1000 C)

1369

0.12 0.1 0.08 0.06 0.04

mPE-REF mPE-ASH mPE-MI mPE-HD mPE-MMD

250 200 150 100 50

0.02 0

0 0

30

60

90

120

mPE-REF mPE-ASH mPE-MI mPE-HD mPE-MMD

where [trans-vinylene]0 is the initial trans-vinylene concentration and [trans-vinylene]t is the trans-vinylene concentration at mastication time t. Results are presented in Fig. 10. The percentage change in trans-vinylene versus time response for the mPEs can be split into three classes. mPE-HD and mPE-MMD showed a low limiting value of percentage change in trans-vinylene after ca. 15 min whereas the mPE-MI showed a higher limiting value after ca. 30 min. mPE-ASH and mPE-REF showed an ‘S’ shaped response up to 240% with an inflexion between ca. 50 and 80 min. trans-Vinylene group formation is a process characteristic of reactions of secondary radicals at low oxygen pressure. The mechanism of formation can be explained by disproportionation reaction between a primary and secondary alkyl radical [18] (Reaction (2)). CH2 + R'

CH

CH2

R CH3 + R'

R"

CH CH

R"

(2)

trans - vinylene

Another possibility for the formation of trans-vinylene group is a b-scission of a secondary radical to a branch point (Reaction (3)) [18]. R

CH

CH

R'

CH

R

+

R'

CH

CH2

(3)

R"

trans - vinylene

CH2 R"

Finally, the isomerisation reaction of vinyl groups that yields trans-vinylene groups has also been considered for the formation of trans-vinylene unsaturation [18] (Reaction (4)). R + R'

CH2

CH

R

CH2

H

+ R'

CH

CH

CH

60

90

120

CH

(4) CH3 +

trans - vinylene

.R"

R"

R'

CH CH

Although all reactions may probably occur, the isomerisation reaction may have an important role in the initial stage of degradation of mPE-REF and mPE-ASH during mastication in air. Indeed, the excessive reduction in vinyl content observed during the early stages of mastication of both polymers could be explained by such an isomerisation reaction. Therefore, the dramatic decay in vinyl content observed for both mPE-REF and mPE-ASH may not only be due to addition of alkyl radicals to vinyls, but also to competitive isomerisation of vinyl unsaturation. At more advanced stages of processing degradation, another more important mechanism was considered to be responsible for the rapid growth of trans-vinylene unsaturation in mPEREF and mPE-ASH particularly. First of all, significant chain scission reactions were observed from the torqueetime profile of both polymers at more extended mastication periods (Figs. 1 and 2). It is significant that chain scission reactions were more pronounced with mPE-ASH. b-Scission of secondary radicals to a branch point may therefore play a significant role in the prolific formation of trans-vinylene groups (Reaction (3)) at more advanced stages of mastication. Furthermore, discolouration was strongly apparent for mPE-ASH only at 120 min of mastication in air. Colour development was not apparent in the cases of the other samples during mastication. These results are not surprising since it is well recognised that colour development increases with catalyst residue content [20]. The discolouration observed with mPE-ASH (high catalyst residue content) is therefore entirely consistent with this well-established trend. However, discolouration has also been attributed to the formation of both trans-vinylene unsaturation and carbonyl groups [21]. Indeed, the presence of both oxygen and structural groups changes are required to produce discolouration during thermal oxidation of polyolefins [18,21].

CH2

vinyl

R'

30

Fig. 10. Changes of trans-vinylene unsaturation groups, expressed in percentage of initial vinyl unsaturation content, in mPEs masticated in air at 225
C and 10 rpm in a BrabenderÒ mixer.

Fig. 9. Evolution of trans-vinylene unsaturation groups of mPEs masticated in air at 225
C and 10 rpm in a BrabenderÒ mixer.

R

0

Mastication Time (min)

Mastication Time (min)

.

CH2

3.5. Evolution of carbonyl groups during mastication Carbonyl Index data are presented in Fig. 11. As expected, mPE-ASH showed the greatest carbonyl formation. mPE-REF

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free radicals formed from hydroperoxide decomposition [24,25]. The formation of shear induced free radicals was found to be dependent on polymer melt viscosity [24]. This would explain why both polymers having relatively lower melt viscosity, i.e., mPE-MI and mPE-MMD, exhibited lowest initiation rate of oxidation during processing in the open mixer.

0.080 mPE-REF mPE-ASH mPE-MI mPE-HD mPE-MMD

Carbonyl Index

0.070 0.060 0.050 0.040 0.030 0.020

3.6. Evolution of hydroperoxide concentration during mastication

0.010 0.000 20

40

60

80

100

120

Mastication Time (min) Fig. 11. Carbonyl Index of mPEs masticated in air at 225
C and 10 rpm in a BrabenderÒ mixer.

also exhibits high carbonyl concentration at 120 min of mastication. However, the carbonyl and trans-vinylene levels may not have reached their ‘‘critical’’ values for the promotion of discolouration at this stage of mastication. Furthermore, the presence of high levels of catalyst residues may also be a significant factor for the formation of colour. mPE-HD was found to exhibit a similar rate of carbonyl formation as mPE-REF. Nevertheless, mPE-HD exhibited a greater initiation rate of oxidation compared with mPE-MI and mPE-MMD. Actually, the greatest oxidative stability was recorded for mPE-MI, where carbonyl index reached a low limiting value during the early stages of mastication. This corroborates quite well with the literature [3] that highlighted a relationship between melt flow and oxidative degradation during processing; lower melt flow grades of LDPE showed higher initial rates of melt oxidation and higher melt flow grades showed greater melt stability [3]. A similar but less defined trend was also observed for LLDPE masticated at 150
C. Discrepancies in the latter trend were attributed to the possible presence of oxidationcatalyst metal impurities. However, it was suggested that the difference between the polyethylene types could not be attributed solely to the melt flow. The broadness of the molar mass distribution curve was also suggested to be another important factor influencing melt stability. The mPE-MMD also showed relatively good oxidative stability during mastication in air; its initiation rate of carbonyl growth was rather low initially, and only increased significantly after 90 min of mastication. PEs with narrow molar mass distribution (MMD) are well known to exhibit higher melt viscosity. A study comparing the influence of molar mass distribution of different polyolefins indicated that melt viscosities of broader MMD resins showed greater shear rate sensitivity [22]. Consequently, broad MMD grades required reduced specific energy inputs for extrusion than the narrow MMD grades of the same MI. This confirms the previous observations regarding the relatively lower torque profile recorded for mPE-MMD relative to the narrow molar distribution mPEs (apart from mPE-MI). Furthermore, initiation of oxidation degradation in mixers has been attributed to the formation of free radicals by mechanical breakdown of the polymer chains by shear, rather than by

The hydroperoxide stability of the metallocene polyethylenes investigated was also assessed as a function of mastication time (Fig. 12). All hydroperoxide versus mastication time curves fit to a third order polynomial equation ((ROOH) Z a Cbt C ct2 C dt3, where a, b, c and d are parameter functions of the polymer and the processing conditions), as it has been demonstrated in the literature [3,23]. The poor fit can be explained by the large standard error that is typical for the iodometric method, which is not a precise technique. It is evident that hydroperoxide concentration increase to a maximum then decreases and then shows another increase. Assuming that the iodometric analysis is a measure of the sum of all hydroperoxides (free and associated hydroperoxides) present in the polyethylene samples, the decrease in hydroperoxide concentration is likely to be attributed to decomposition of hydroperoxides whereas the increase after this decrease could be probably caused by the formation of associated hydroperoxides, as suggested in the literature [3,23]. Indeed, it has been demonstrated that during mastication of LDPE in air at 150
C, the concentration of associated hydroperoxides increased proportionally with the concentration of free hydroperoxides at short processing times, and increased more and more rapidly at more extended processing periods, whilst the concentration of free hydroperoxides decreases [23]. Looking closely at the data, a striking feature could be observed regarding initiation rates of hydroperoxide formation.

0.8 mPE-REF mPE-ASH mPE-MI mPE-HD mPE-MMD

0.7

[ROOH] (mmol/g)

0

0.6 0.5 0.4 0.3 0.2 0.1 0.0 0

30

60

90

120

Mastication Time (min) Fig. 12. Hydroperoxide concentrations in mPEs masticated in an open mixer at 225
C and 10 rpm.

E.M. Hoa`ng et al. / Polymer Degradation and Stability 91 (2006) 1363e1372

A lower initial rate of hydroperoxide formation was recorded for both mPE-MI and mPE-MMD relative to mPE-REF and mPE-ASH. The lower melt viscosity of mPE-MI and mPEMMD relative to the other samples has been pointed out in the previous section. It may therefore be argued that lower melt viscosity affords a reduced amount of mechano-oxidative degradation due to reduced entanglements and hence chain scission events. This is in agreement with other published processing degradation studies on LDPE, HDPE and LLDPE [3,24]. It was found that the initial rate of hydroperoxide formation decreases with increasing melt flow of various types of polyethylene (LDPE, HDPE and LLDPE) masticated in air [24]. It is significant that hydroperoxide formation has been explained by primary initiation of free radicals formation [25]. The relationship between crystalline content and hydroperoxide formation has been discussed in the literature where it is concluded that hydroperoxides are formed in the amorphous phase [23]. The latter trend was also observed in this study where mPE-HD displayed the lowest level of hydroperoxide formation.

4. Conclusion It has been found that the mPE samples investigated showed different behaviour during mastication in air at 225
C and 10 rpm. With the exception of mPE-MMD, crosslinking was found to dominate at the early stages of degradation during mastication. Crosslinking reactions were found to be initiated at points of vinyl unsaturation. Addition reactions of alkyl radicals to vinyl groups was considered to be the dominant reaction as a decline in vinyl concentration was observed simultaneously with a reduction in MFR as mastication time increased. However, isomerisation reaction of vinyl to yield trans-vinylene unsaturation was also thought to occur with mPE-REF and mPE-ASH at early stages of mastication. This theory was advanced in order to explain the difference in rate of reduction in vinyl concentration with mPE-HD, which exhibited a similar MFR reduction profile and lower rate of trans-vinylene formation. Furthermore, discolouration was only observed for mPE-ASH after 120 min of mastication in air. Colour formation was attributed to both excessive levels of catalyst residues and extensive formation of conjugated systems consisting of trans-vinylene and carbonyl groups. Finally, it was concluded that the polymer melt viscosity, i.e., molar mass and shape of molar mass distribution, appeared to govern the processing stability of the mPE samples investigated. Consequently, the most stable mPE sample provided was the polymer having the highest melt flow, i.e., mPE-MI. This sample exhibited best retention of MFR, as well as best oxidative stability, as manifested by the lowest carbonyl formation during mastication in air. The width of the molar mass distribution curve was also found to be an important factor influencing the initiation stages of oxidative degradation in the melt state. The low molar mass components of mPE-MMD (with wide molar mass distribution) reduced the shear on the longer chain in the melt during mastication via a plasticisation effect, therefore, unsurprisingly, mPE-MMD also showed

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relatively good MFR retention and oxidative stability during processing in the open mixer. These results confirm the importance of shear as the major source for initiation of free radicals formed by homolytic fission caused via mechanical cleavage of polymer chains.

Acknowledgements The author would like to thank REPSOL-YPF for supplying the polymer and for their financial support.

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