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Qualification of pipe-grade HDPEs: Part I, development of a suitable accelerated ageing method
Polymer Testing 28 (2009) 96–102
Contents lists available at ScienceDirect
Polymer Testing journal homepage: www.elsevier.com/locate/polytest
Test Method
Qualification of pipe-grade HDPEs: Part I, development of a suitable accelerated ageing method Tania Zanasi a, b, Elena Fabbri a, b, Francesco Pilati a, b, * a b
Department of Materials and Environmental Engineering, University of Modena and Reggio Emilia, 41100 Modena, Italy Italian Consortium for Science and Technology of Materials (INSTM), Florence, Italy
a r t i c l e i n f o
a b s t r a c t
Article history: Received 3 September 2008 Accepted 4 November 2008
Several techniques of polymer characterization and different ageing methods have been used with the aim of developing a simple, fast and reliable method to qualify commercial pipe-grade polyethylene samples, and possibly to evidence the presence of recycled PE within PE pipes. The results of the different techniques used have been compared with respect to their capability to evidence differences in the degradation rate of different HDPE samples (including virgin HDPE, HDPE pipes obtained from virgin HDPE and HDPE pipes that probably contain recycled HDPE). FT-IR, TGA and DSC were found unsuitable for this purpose but, on the contrary, MFI measurements have been found sensitive enough to evidence different degradation rates when a suitable combination of high temperature, oxygen, mechanical stresses and mixing time had been used for ageing the sample. Ó 2008 Elsevier Ltd. All rights reserved.
Keywords: Polyethylene Accelerated ageing tests Recycled polyethylene Pipe degradation HDPE ageing
1. Introduction High density polyethylene (HDPE) is the most widely used plastic for the production of pipes for water and gas piping applications. One of the most important features required in PE pipes is durability, as failure during service life results in costly maintenance operations and in consumer discomfort. Premature failure is often related to changes in the PE properties induced by degradation reactions which can lead to a decrease of mechanical properties and to unexpectedly fast failure. Several factors can contribute to a fast degradation of PE pipes. In particular, the presence/formation of chemical moieties that undergo fast degradation, such as peroxide and hydroperoxide groups, is particularly harmful. Their presence/ formation in polymer samples is related to the previous thermo-mechanical history and service-life conditions. During extrusion, a first thermo-mechanical degradation step can occur, the extent of which depends on processing * Corresponding author. Tel.: þ39 (0) 59 205 6213; fax: þ39 (0) 59 205 6243. E-mail address: [email protected] (F. Pilati). 0142-9418/$ – see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.polymertesting.2008.11.006
parameters. Continuous contact with oxygen and other external agents (weathering, biological organisms) can also make a significant contribution to the progress of degradation reactions during the service life. Liquids or gases flowing in the pipes may be an additional factor that can reduce the PE-pipe resistance to degradation, as they can remove stabilizer additives from the internal surface making PE more sensitive to degradation [1,2], particularly in the presence of aggressive chemicals, for instance chlorine, which can more easily attack the unprotected PE surface. The ‘‘oxidative degradation’’ of PE has been widely studied [3–10]; many different reactions involving free radicals can occur and, according to their relative rates, they can lead mainly to chain scission or branching and crosslinking. While the effects of temperature and oxygen are generally recognized, there is still a debate about the role of mechanical stresses [3,6,7,11]. It is generally believed that HDPE shows a higher tendency to crosslinking rather than to molecular weight reduction, however Rideal and Padget [3] concluded that both chain scission and crosslinking are concurrently influenced by shear stresses, oxygen and temperature.
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In recent years, a continuously increasing amount of recycled PE has been made available in the market at low cost, and somebody may be tempted to use recycled PE to replace, completely or in part, the more expensive virgin PE. In this case, initial flow and mechanical properties may be adjusted to be the same or very close to those of virgin PE; however, the degradation rate of recycled plastics is expected to be faster, as a consequence of the degradation reactions that occurred during the previous service life, and unexpectedly premature failure in service may occur [1,2]. In fact, due to the contact with oxygen and other chemicals, modifications in the molecular structure can occur, and recycled PE is expected to contain a concentration of hydroperoxide and peroxide groups significantly higher than that of virgin PE. The accumulated oxygen-containing groups makes HDPE more susceptible to degradation and can substantially modify the thermal stability and durability of the recycled plastics [1,12]. Recovery and recycling operations on post-consumer plastic wastes can further increase the concentration of oxygen-containing groups in recycled plastics. Therefore, when recycled PE is used to produce PE pipes (alone or mixed with virgin PE) degradation reactions are expected to be faster (during both processing and service life) and pipe durability to be shorter. For all these reasons, it would be important to set up a method able to estimate the resistance to oxidation of PE used to produce PE pipes and, possibly, to distinguish between products containing only virgin PE from those containing also recycled PE. Spectroscopic techniques (FT-IR, mainly, and NMR) have been proposed as analytical tools to detect the progress of degradation reactions [8,13], however, they require the availability of both expensive facilities and skilled technicians for an appropriate manipulation of the samples and interpretation of the results of the complex phenomena involved. These conditions are rarely fulfilled in small or medium size factories that produce or use pipes, and the development of a practical method able to estimate the quality of PE with respect to degradation rate is still a challenge that would give answer to industrial requirements. In this context, the present work is an attempt to provide a simple, fast, economic and reliable method for qualification purpose of pipe-grade PE materials. It is based on the consideration that good quality virgin HDPEs can resist severe degradation conditions better than low quality ones (second rate virgin, recycled or postconsumer). Several investigations on PE samples, including granules and pipes of good quality derived from virgin HDPE, and a poor quality PE pipe, presumably containing recycled PE, have been performed in part I of this work, using the most commonly employed techniques to study HDPE degradation and different ageing methods to differentiate the degradation behaviour of the samples. The results from different ageing methods and characterization techniques have been compared for their capability to evidence differences in the PE resistance to degradation. In a second paper, the part II of this work, the most efficient ageing method is used to derive a Resistance to Oxidation Index (ROI), which will be proposed as a possible
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criterion for material ranking and to discriminate virgin PE from samples containing recycled PE. 2. Materials and methods 2.1. Materials Commercial pellets of virgin HDPE (PE100VG and PE80VG) and two PE pipes (one extruded from PE100 pellets, PE100VP, and another one whose exact origin is unknown and that is believed to contain recycled PE, PE100UP), have been considered in this paper. The information available for the samples is summarized in Table 1. In order to perform tests, HDPE pipes were ground in a granulator (Piovan, Italy), and the collected granules (3– 4 mm average dimension) were used for characterization. 2.2. FT-IR analysis ATR FT-IR spectra were recorded using a spectrometer Avatar 330 FT-IR Thermonicolet. 2.3. Thermogravimetric analysis (TGA) TGA measurements were performed on a Perkin Elmer TGA7. Approximately 10 mg of sample was heated at 30 C/min heating rate from 25 C to 800 C under a gas-stream flow of either air or nitrogen. 2.4. Differential scanning calorimetry Differential scanning calorimetry (DSC) was performed with a Thermal Analysis DSC 2010 TA Instrument. Non isothermal crystallization curves were recorded by cooling samples from the melt (200 C) at different cooling rates (10, 20 and 30 C/min) under nitrogen atmosphere. Isothermal crystallization curves were recorded under nitrogen at 121 C over 60 min, after rapid cooling from 200 C. DSC heating curves were also recorded at 20 C/min. 2.5. Melt flow index measurements (MFI) MFI was measured by using a CEAST Melt-Index model 6540/0000 instrument, in accordance with ISO 1133:1997 (the test temperature was set at 190 C and the nominal load was 5 kg). During each measurement, samples were taken at different times and the initial MFI was obtained by Table 1 Sample’s characteristics. Sample code
Description
MFIa (g/10 min)
Carbon black Contentb (wt %)
PE100VG PE80VG PE100VP PE100UP
Pellets of virgin HDPE Pellets of virgin HDPE Pipe made with PE100VG Pipe, which probably contains an unknown amount of recycled PE
0.18 0.86 0.20 0.65
2.45 2.95 2.47 4.20
a b
0.02 0.01 0.02 0.01
Measured according to ISO 1133:1997. Residue from TGA up to 800 C under nitrogen.
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extrapolating the recorded values to zero-time. Measurements were repeated at least four times for each sample.
2.6. Thermal ageing PE100VG, PE100VP and PE100UP samples were submitted to accelerated thermal ageing within both the MFI chamber (at 190 C, for 4, 8 and 16 h) and within an internal mixer (IM) (Haake Rheomix PolyLab System) at 190 C under different rotor speeds, mixing times and with different fillings of the IM chamber. The conditions used for the different ageing methods in IM are summarized in Table 2. 3. Results 3.1. Characterization of unaged samples The most powerful technique to characterize and to reveal changes in molecular structure of HDPE (branching, double bonds, peroxide moieties, etc.) is probably NMR spectroscopy (1H- and 13C- both in solution and in the solid state) [13,14]. However, it requires expensive instruments not easily available for routine measurements and skilled technicians able to derive information from the complex spectra. IR spectroscopy has been widely used to characterize PE samples and, in particular, to detect the presence of various oxidation products that are formed as a consequence of degradation reactions in the presence of oxygen [3,5,7,8]. In principle, a quantitative evaluation of peroxide and hydroperoxide groups could be used as an index of the tendency of HDPE to undergo degradation in service; however, the relatively low rate of formation of these groups in HDPE [1,5,7] and the common presence of additives and carbon black in commercial PE-pipes can reduce the sensitivity of this technique and make difficult to derive information from such measurements. In order to characterize the samples listed in Table 1 with respect to the content of chemical moieties that can behave like degradation promoters, ATR-FT-IR spectra have been recorded. No significant differences appeared in the spectra and in particular no evidence of specific bands in the wave-number regions 1600–1800, 800–1100 and 3200–3700 cm1, typical of carbonyl and carboxyl, vinyl and vinylidene, and hydroxyl and hydroperoxide groups,
respectively [5,8], were observed even in PE100UP, which was expected to contain recycled PE. TGA has also been proposed as a possible technique to investigate the HDPE degradation [15]; as the rate of weight loss is expected to be related to the rate of formation of low molecular weight fragments from degradation reactions. For all the samples listed in Table 1, TGA curves were recorded at 30 C/min up to 800 C, under a stream of either N2 or air. Under air, no residual weight was present at 800 C; therefore, the residual weight remaining at 800 C under N2 can be reasonably attributed to the carbon black (initially contained or formed by pyrolysis of HDPE). The residual weight percentage at 800 C under N2 is reported in Table 1. As it can be seen, the residual weight percentages are the same for virgin PE samples (PE100VG, PE100VP, PE80VG) and significantly higher for sample PE100UP. Even though the real origin and composition of sample PE100UP are unknown, this result supports the suspicion that it contains recycled PE, as it is reasonable to think that a higher carbon black content was added to improve the initial mechanical properties of the HDPE pipes. Of course, this evidence cannot be considered conclusive with respect to the qualification of HDPE. Fig. 1 shows TGA curves recorded under N2 and air. The temperature at which there is the maximum rate of weight loss can be taken as an indication of the resistance to degradation. As expected, the degradation occurs faster under air than under nitrogen, the maximum weight-loss rate occurring at lower temperatures for both PE100VG and PE100UP. The maximum weight loss rate for PE100UP occurs at lower temperature with respect to PE100VG both under air and nitrogen, suggesting a lower resistance to degradation for PE100UP. This last result supports the hypothesis that sample PE100UP contains recycled PE and suggests that TGA is able to evidence a faster degradation rate for sample PE100UP with respect to virgin PE100VG. However, it has also to be considered that the shape and position of TGA curves can depend on the initial molecular weight of the samples, and a significant contribution to the difference between the temperature of maximum weight
Table 2 Operating conditions for thermo-mechanical-oxidative ageing in the internal mixer at 190 C. Method Feeding Rotor speed Mixing PE100VG PE100VP PE100UP weighta (g) (rpm) time (h) A B C D Eb
54 45 45 35 45
30 30 30 30 10–100b
4 4 1 1 1
X X X X X
X X
X X X X X
a
IM chamber is completely filled with 54 g of PE; as the IM chamber was open to air, the amount of oxygen within the mixing chamber was dependent on the sample weight feeded. b Various rotor-speeds, from 10 to 100 rpm, have been used.
Fig. 1. TGA curves recorded at 30 C/min heating rate for samples PE100VG, PE80VG and PE100UP, either under nitrogen (N2) or under air.
T. Zanasi et al. / Polymer Testing 28 (2009) 96–102
loss rate of samples PE100UP and PE100VG may derive from the significantly lower molecular weight of sample PE100UP rather than from the different degradation rate. Indeed, if we compare the TGA curves of sample PE100VG and PE80VG, both from virgin samples but with different initial MFIs, it is evident that there is a significant shifting to lower temperature for sample PE80VG with a higher MFI. Furthermore, the curve obtained for the virgin PE80VG is close to that of PE100UP, and it can be concluded that it is not easy to separate the effects of degradation rate from those of molecular weight. As a consequence, TGA seems not suitable for the aim of this work. DSC has also been used to investigate HDPE degradation [1,2,14,16]. In principle, changes in the chemical structure occurring upon degradation, such as changes in molecular weight and/or formation of branching, could induce changes in crystallization and melting behaviour that could be recorded by DSC. For all samples, DSC curves were recorded either under cooling (from the molten state at different cooling rates), under isothermal conditions (121 C) and under heating at 20 C/min. The most relevant data are summarized in Table 3. Just small differences appear between different samples and they cannot be easily related to the degradation rate behaviour expected according to the previous thermo-mechanical history of the samples. Finally, to complete the characterization of the as received samples, MFI was measured according to ISO 1133:1997. Extruded samples were collected from the MFI apparatus every 10 min over 1 h. MFI data are reported vs time in Fig. 2. As it appears, there is a slight increase of MFI for samples taken at increasing times, within 1 h, suggesting that degradation occurs during the measurement. Initial values of MFI were therefore obtained by extrapolation to zero-time and are reported in Table 1. As expected, the MFI values are very similar for samples PE100VG and PE100VP, whereas MFI is significantly higher for PE100UP. 3.2. Degradation upon conventional ageing One of the most used methods to investigate durability of polyolefins is to perform accelerated embrittlement tests on solid samples aged in an oven at high temperature (typically just below the melting temperature). Extrapolation to low temperature is then used to get information about durability during service life. However, this ageing method requires several weeks or months to give a response and it is not suitable for the aim of this work. It is expected that the higher the ageing temperature the faster is the degradation rate, therefore, in order to achieve information about the degradation rate in a relatively short Table 3 DSC data of as received and IM aged samples recorded under crystallization at 20 C/min. Sample code
PE100VG PE100VP PE100UP
As received (unaged)
Aged
Tc,peak ( C)
DHc (J/g)
Tc,peak ( C)
DHc (J/g)
106.1 102.2 104.9
137.4 142.5 133.8
109.2 103.5 104.0
135.6 147.2 132.0
99
Fig. 2. MFI of PE100VG pellets and pipes and PE100UP pipe at 190 C.
time, the samples listed in Table 1 were submitted to conventional accelerated ageing at 190 C, in the molten state, within the MFI chamber (for times up to 16 h). Aged samples were then submitted to FT-IR, TGA, DSC and MFI characterization in order to investigate the changes induced by ageing. Neither FT-IR nor TGA and DSC gave significant changes after ageing. On the contrary, some significant differences were observed in MFI measurements. Fig. 2 shows the MFI data recorded up to 260 min for samples heated under shear (from 0 to 70 min and from 220 to 280 min) or under static heating (from 70 to 220 min). As it can be seen, MFI slightly increases under shear, whereas a slight decrease of MFI is observed after static heating. These results are in agreement with other ones previously reported in the literature [3], and suggest that when mechanical stresses are applied the degradation mechanism changes with respect to that occurring during static heating. If we consider that both chain scission and branching reactions can occur concurrently at the same time, MFI results suggest that chain scission prevails over branching/crosslinking under shear, whereas the contrary seems to happen under static heating. In order to understand if the MFI decrease can be used to investigate the durability characteristics of different samples, MFI tests were performed on samples submitted to static heating for various times (4, 8 and 16 h); the results are reported in Table 4. Changes in MFI are very limited (values are almost unchanged) for samples PE100VG and PE100VP, suggesting either a low degradation rate or compensation of opposite Table 4 MFI values measured after different heating times in the MFI chamber at 190 C. Ageing time (h)
PE100VG MFI (g/100 )
PE100VP MFI (g/100 )
PE100UP MFI (g/100 )
0 4 8 16
0.18 0.176 0.20 0.20
0.20 0.18 0.18 0.20
0.65 0.52 0.51 0.56
0.02 0.004 0.01 0.02
0.02 0.02 0.01 0.01
0.01 0.02 0.03 0.04
100
T. Zanasi et al. / Polymer Testing 28 (2009) 96–102
effects such as those that would be originated by chain scission and branching. More significant changes were obtained for sample PE100UP; a relatively fast decrease of MFI after 4 h of ageing, is followed by a slight increase over longer times. These results suggest different behaviour towards degradation of samples PE100VG and PE100UP, but the effect of ageing is too limited and therefore unsuitable for the development of a ranking criterion for unknown HDPE samples. 3.3. Degradation upon ageing in internal mixer Due to the limited effects observed upon ageing under static heating in the MFI chamber at 190 C, it was decided to move to stronger ageing procedures, able to induce more extended changes in the samples in a relatively short time. According to the literature [3] and in agreement with the above reported MFI results, degradation phenomena are expected to be faster when PE samples are submitted to melt mixing in the presence of oxygen and under mechanical stresses. In order to have all these factors operating at the same time, samples were melt blended in an internal mixer (IM) at 190 C for various times, using different rotor speeds and with the IM chamber either completely (54 g) or partially filled (45 and 35 g) with samples (see Table 2). After melt mixing, the recovered samples were ground and investigated by FT-IR, DSC, and MFI. Again, no significant changes were observed in FT-IR spectra and in DSC curves, while relevant changes were observed in MFI. Fig. 3 shows DSC traces before and after
ageing (according to method C), as an example. It is interesting to note that small changes were found for PE100VG and PE100VP, whereas almost identical curves were found for sample PE100UP, which, on the contrary, was expected to undergo a more extended degradation and, therefore, to show the strongest changes. Results of MFI measurements are reported in Table 5. As can be seen, limited changes were observed after ageing at 30 rpm for 4 h, when the IM chamber was completely filled (ageing method A). On the contrary, very significant changes were observed when the amount of sample in the IM chamber was reduced to 45 g (ageing method B). Significant changes were also observed with the same amount of sample (45 g) for a shorter time (1 h, ageing method C). Instead, limited changes were observed by further reducing the amount of sample to 35 g (ageing method D). It is evident that the amount of sample charged in the IM chamber plays a relevant role. This is not surprising, as a reduced amount of sample in the IM chamber is expected to affect both the volume of air within the IM chamber (the lower is the polymer charged the higher is the volume of air in the IM chamber and, therefore, the amount of oxygen that is in contact with the polymer during melt mixing), and the nature and intensity of the stresses acting on the molten polymer. When the IM chamber is completely filled, the mechanical stresses are expected to be the highest, but the amount of oxygen that is in contact with the polymer is limited. On the other hand, when a too small amount of sample is charged in the IM chamber (35 g) the friction of the molten polymer with the chamber walls is reduced and the polymer is expected to
Fig. 3. DSC crystallization curves recorded at 20 /min after melting at 200 C for samples as received and after ageing by method C (sample/a).
T. Zanasi et al. / Polymer Testing 28 (2009) 96–102
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Table 5 MFI values (and percent change with respect to unaged samples) of PE100VG, PE100VP and PE100UP samples after different thermo-mechanical ageing treatments in the internal mixer chamber at 190 C and 30 rpm. Ageing in the IM chamber Method A B C D
PE100VG MFI (g/100 )
DMFI (%)
MFI (g/100 )
DMFI (%)
MFI (g/100 )
DMFI (%)
0.18 0.26 1.05 0.27 0.24
–
0.20 0.02
–
0.65 0.83 3.6 1.09 0.99
–
54 45 45 35
0 4 4 1 1
0.02 0.02 0.04 0.01 0.01
4. Discussion and conclusions Considering that premature failure of PE pipes for water and gas pipe applications should be avoided as it causes consumer’s discomfort and requires costly maintenance operations, it is obvious that a method able to predict durability would be highly appreciated. Durability of PE pipes is expected to be strongly related to the rate of degradation reactions occurring during service life and an accelerated testing method able to predict a too fast degradation in service would be highly desirable. It should be reliable, fast and simple, so that it can be carried out in small/medium size factories where PE pipes are produced or used, by people not particularly scientifically skilled and using relatively inexpensive apparatuses. Accelerated ageing at high temperature is a common practice used to increase the rate of degradation reactions in order to reduce the experimental time required to obtain relevant changes in some properties and information about the useful service life. However, the commonly used ageing methods (static heating of solids and repeated extrusions) Table 6 Effect of rotor speed on MFI for samples submitted to melt mixing at 190 C for 1 h, in a partially filled (45 g) IM chamber. Sample code
Rotor speed (rpm)
MFI (g/10’)
PE100VG
10 30 50 100 10 30 50 100 10 30 50 100
0.19 0.26 0.72 2.92 – 0.30 0.78 3.13 – 1.10 – 2.40
PE100UP
PE100UP
Mixing time (h)
stick to the rotors and to move integrally with them, with limited or no stresses (both elongational and shear stresses) acting on the polymer melt. The effect of rotor speed over 1 h mixing has also been investigated in a partially filled IM chamber, as appears from the data of Table 6. An increase in the rotor speed from 10 to 100 rpm leads to a progressive increase in MFI for all samples, however, the MFI increase registered for sample PE100UP is lower than for PE100VG and PE100VP, suggesting a different degradation mechanism.
PE100VP
PE100VP
Feeding weight (g)
9 349 17 4
0.30 0.02
36
0.01 0.05 0.2 0.06 0.05
22 432 60 45
require either too long times or are almost ineffective, in particular when HDPE contains stabilizers [7]. There is much evidence in the literature suggesting that the relative rate of the various degradation reactions may be different under different ageing conditions and, in particular, it has been reported that a different reaction mechanism and synergistic effects may derive from the simultaneous combination of different factors such as temperature, oxygen concentration and mechanical stresses [3]. For example, when HDPE is subjected to a shear stress, the degradation is mainly due to primary reactions, i.e. chain scission of polymer chains rather than reactions involving oxidation products accumulated in the system. Nevertheless, previous studies on HDPE ageing have mainly dealt with factors able to accelerate degradation reactions separately, typically by submitting HDPE samples either to repeated extrusions or to static oxidative oven ageing, and only seldom has a combination of these factors been used [3]. On the other hand, the results of this study have shown that ageing in IM under a suitable combination of temperature, oxygen concentration, time and rotor speed can be effective, leading to a strong degradation of HDPE sample in a relatively short time. To demonstrate the effects of accelerated ageing of HDPE, various techniques are available. Embrittlement of solid specimens, changes in FT-IR and NMR spectra, different evolution rates of low molecular weight by products on heating (TGA, GC–MS), changes in crystallization rate and in the extent of crystallization and in rheological properties are the expected results of ageing. All these techniques have been extensively used to investigate PE degradation. Embrittlement tests, one of the most used methods to derive information about durability, require ageing times from several weeks to few months, too long a time for many purposes. On the other hand, spectroscopic methods (NMR and FT-IR), the more appropriated techniques for scientific investigation of the degradation mechanism, require costly instruments, skilled personal and time consuming calibrations, and the results are not easily related to durability. The presence of additives and carbon black in commercial samples is a further complication that requires long and expensive operations in order to obtain samples suitable to record clear spectra. As reported in the literature, changes in the spectra of HDPE are difficult to observe even after very long ageing times [1,5]. Spectroscopic techniques (FT-IR) and thermal characterization (DSC and TGA) have been found unsuitable for the purpose
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of this article when applied to commercial pipe-grade HDPE. On the contrary, MFI proved to be effective under suitable ageing conditions (method C in particular) and it was possible to differentiate the degradation behaviour of virgin HDPE granules (PE100VG) from good-quality HDPE pipes (PE100VP) and from poor-quality pipes (PE100UP). Indeed, melt flow is strongly affected by the molecular structure (molecular weight, MWD, presence of branching) and, therefore, it is not surprising that MFI measurements have been found the most sensitive to reveal the effects of ageing among the various techniques used. The combined effect of temperature, stresses, oxygen and residence time, such as method C, seems to be a very effective ageing method able to give interesting results for HDPE qualification when combined with MFI measurements. Too long ageing times lead to too strong increases of MFI, the effect of degradation are so high that the discrimination capability between samples PE100VG and PE100UP is reduced. An increase of mechanical stress by increasing rotor speed (compare tests in Tables 5 and 6) has similar effects; a too high rotor speed leads to too rapid degradation that reduces the difference in MFI changes for different samples and makes it difficult to rank HDPE samples for their resistance to oxidation. A too high increase of the volume of air in the IM chamber is more than counterbalanced by a too strong decrease of mechanical stress, with a significant reduction of the overall effect of ageing on MFI (compare tests C and D). On the other hand, a too small amount of air in the IM chamber reduces the effects of ageing, presumably because of the smaller amount of oxygen able to come in contact with PE chains (compare sample A and B). By comparing the results obtained for samples PE100VG, PE100VP and PE100UP after ageing with method C, we can observe that the MFI increase is 17%, for PE100VG (virgin HDPE), 30% for PE100VP (pipes from virgin HDPE) and 60% for PE100UP (granules obtained from a pipe that probably was prepared by using also recycled PE). Thus, ageing method C seems able to rank samples for their resistance to degradation, as expected according to their previous thermo-mechanical history. In particular, it is interesting to note that Method C is very sensitive, being able to detect even the effects of an additional extrusion step on the degradability of virgin HDPE (compare MFI values for PE100VG with those for PE100VP). Even though more tests on a larger number of PE samples are necessary to definitely conclude that ageing
method C is a general one applicable to rank PE pipes for their durability (various PE sample are under investigation), we can conclude that it is a simple, easy and fast method able to demonstrate differences in the degradation behaviour of commercial HDPE samples. Finally, it has to be noted that both IM and MFI are apparatuses that can be easily managed by people not particularly scientifically skilled, and that these apparatuses are frequently present in small/medium size factories.
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Test Method
Qualification of pipe-grade HDPEs: Part I, development of a suitable accelerated ageing method Tania Zanasi a, b, Elena Fabbri a, b, Francesco Pilati a, b, * a b
Department of Materials and Environmental Engineering, University of Modena and Reggio Emilia, 41100 Modena, Italy Italian Consortium for Science and Technology of Materials (INSTM), Florence, Italy
a r t i c l e i n f o
a b s t r a c t
Article history: Received 3 September 2008 Accepted 4 November 2008
Several techniques of polymer characterization and different ageing methods have been used with the aim of developing a simple, fast and reliable method to qualify commercial pipe-grade polyethylene samples, and possibly to evidence the presence of recycled PE within PE pipes. The results of the different techniques used have been compared with respect to their capability to evidence differences in the degradation rate of different HDPE samples (including virgin HDPE, HDPE pipes obtained from virgin HDPE and HDPE pipes that probably contain recycled HDPE). FT-IR, TGA and DSC were found unsuitable for this purpose but, on the contrary, MFI measurements have been found sensitive enough to evidence different degradation rates when a suitable combination of high temperature, oxygen, mechanical stresses and mixing time had been used for ageing the sample. Ó 2008 Elsevier Ltd. All rights reserved.
Keywords: Polyethylene Accelerated ageing tests Recycled polyethylene Pipe degradation HDPE ageing
1. Introduction High density polyethylene (HDPE) is the most widely used plastic for the production of pipes for water and gas piping applications. One of the most important features required in PE pipes is durability, as failure during service life results in costly maintenance operations and in consumer discomfort. Premature failure is often related to changes in the PE properties induced by degradation reactions which can lead to a decrease of mechanical properties and to unexpectedly fast failure. Several factors can contribute to a fast degradation of PE pipes. In particular, the presence/formation of chemical moieties that undergo fast degradation, such as peroxide and hydroperoxide groups, is particularly harmful. Their presence/ formation in polymer samples is related to the previous thermo-mechanical history and service-life conditions. During extrusion, a first thermo-mechanical degradation step can occur, the extent of which depends on processing * Corresponding author. Tel.: þ39 (0) 59 205 6213; fax: þ39 (0) 59 205 6243. E-mail address: [email protected] (F. Pilati). 0142-9418/$ – see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.polymertesting.2008.11.006
parameters. Continuous contact with oxygen and other external agents (weathering, biological organisms) can also make a significant contribution to the progress of degradation reactions during the service life. Liquids or gases flowing in the pipes may be an additional factor that can reduce the PE-pipe resistance to degradation, as they can remove stabilizer additives from the internal surface making PE more sensitive to degradation [1,2], particularly in the presence of aggressive chemicals, for instance chlorine, which can more easily attack the unprotected PE surface. The ‘‘oxidative degradation’’ of PE has been widely studied [3–10]; many different reactions involving free radicals can occur and, according to their relative rates, they can lead mainly to chain scission or branching and crosslinking. While the effects of temperature and oxygen are generally recognized, there is still a debate about the role of mechanical stresses [3,6,7,11]. It is generally believed that HDPE shows a higher tendency to crosslinking rather than to molecular weight reduction, however Rideal and Padget [3] concluded that both chain scission and crosslinking are concurrently influenced by shear stresses, oxygen and temperature.
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In recent years, a continuously increasing amount of recycled PE has been made available in the market at low cost, and somebody may be tempted to use recycled PE to replace, completely or in part, the more expensive virgin PE. In this case, initial flow and mechanical properties may be adjusted to be the same or very close to those of virgin PE; however, the degradation rate of recycled plastics is expected to be faster, as a consequence of the degradation reactions that occurred during the previous service life, and unexpectedly premature failure in service may occur [1,2]. In fact, due to the contact with oxygen and other chemicals, modifications in the molecular structure can occur, and recycled PE is expected to contain a concentration of hydroperoxide and peroxide groups significantly higher than that of virgin PE. The accumulated oxygen-containing groups makes HDPE more susceptible to degradation and can substantially modify the thermal stability and durability of the recycled plastics [1,12]. Recovery and recycling operations on post-consumer plastic wastes can further increase the concentration of oxygen-containing groups in recycled plastics. Therefore, when recycled PE is used to produce PE pipes (alone or mixed with virgin PE) degradation reactions are expected to be faster (during both processing and service life) and pipe durability to be shorter. For all these reasons, it would be important to set up a method able to estimate the resistance to oxidation of PE used to produce PE pipes and, possibly, to distinguish between products containing only virgin PE from those containing also recycled PE. Spectroscopic techniques (FT-IR, mainly, and NMR) have been proposed as analytical tools to detect the progress of degradation reactions [8,13], however, they require the availability of both expensive facilities and skilled technicians for an appropriate manipulation of the samples and interpretation of the results of the complex phenomena involved. These conditions are rarely fulfilled in small or medium size factories that produce or use pipes, and the development of a practical method able to estimate the quality of PE with respect to degradation rate is still a challenge that would give answer to industrial requirements. In this context, the present work is an attempt to provide a simple, fast, economic and reliable method for qualification purpose of pipe-grade PE materials. It is based on the consideration that good quality virgin HDPEs can resist severe degradation conditions better than low quality ones (second rate virgin, recycled or postconsumer). Several investigations on PE samples, including granules and pipes of good quality derived from virgin HDPE, and a poor quality PE pipe, presumably containing recycled PE, have been performed in part I of this work, using the most commonly employed techniques to study HDPE degradation and different ageing methods to differentiate the degradation behaviour of the samples. The results from different ageing methods and characterization techniques have been compared for their capability to evidence differences in the PE resistance to degradation. In a second paper, the part II of this work, the most efficient ageing method is used to derive a Resistance to Oxidation Index (ROI), which will be proposed as a possible
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criterion for material ranking and to discriminate virgin PE from samples containing recycled PE. 2. Materials and methods 2.1. Materials Commercial pellets of virgin HDPE (PE100VG and PE80VG) and two PE pipes (one extruded from PE100 pellets, PE100VP, and another one whose exact origin is unknown and that is believed to contain recycled PE, PE100UP), have been considered in this paper. The information available for the samples is summarized in Table 1. In order to perform tests, HDPE pipes were ground in a granulator (Piovan, Italy), and the collected granules (3– 4 mm average dimension) were used for characterization. 2.2. FT-IR analysis ATR FT-IR spectra were recorded using a spectrometer Avatar 330 FT-IR Thermonicolet. 2.3. Thermogravimetric analysis (TGA) TGA measurements were performed on a Perkin Elmer TGA7. Approximately 10 mg of sample was heated at 30 C/min heating rate from 25 C to 800 C under a gas-stream flow of either air or nitrogen. 2.4. Differential scanning calorimetry Differential scanning calorimetry (DSC) was performed with a Thermal Analysis DSC 2010 TA Instrument. Non isothermal crystallization curves were recorded by cooling samples from the melt (200 C) at different cooling rates (10, 20 and 30 C/min) under nitrogen atmosphere. Isothermal crystallization curves were recorded under nitrogen at 121 C over 60 min, after rapid cooling from 200 C. DSC heating curves were also recorded at 20 C/min. 2.5. Melt flow index measurements (MFI) MFI was measured by using a CEAST Melt-Index model 6540/0000 instrument, in accordance with ISO 1133:1997 (the test temperature was set at 190 C and the nominal load was 5 kg). During each measurement, samples were taken at different times and the initial MFI was obtained by Table 1 Sample’s characteristics. Sample code
Description
MFIa (g/10 min)
Carbon black Contentb (wt %)
PE100VG PE80VG PE100VP PE100UP
Pellets of virgin HDPE Pellets of virgin HDPE Pipe made with PE100VG Pipe, which probably contains an unknown amount of recycled PE
0.18 0.86 0.20 0.65
2.45 2.95 2.47 4.20
a b
0.02 0.01 0.02 0.01
Measured according to ISO 1133:1997. Residue from TGA up to 800 C under nitrogen.
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extrapolating the recorded values to zero-time. Measurements were repeated at least four times for each sample.
2.6. Thermal ageing PE100VG, PE100VP and PE100UP samples were submitted to accelerated thermal ageing within both the MFI chamber (at 190 C, for 4, 8 and 16 h) and within an internal mixer (IM) (Haake Rheomix PolyLab System) at 190 C under different rotor speeds, mixing times and with different fillings of the IM chamber. The conditions used for the different ageing methods in IM are summarized in Table 2. 3. Results 3.1. Characterization of unaged samples The most powerful technique to characterize and to reveal changes in molecular structure of HDPE (branching, double bonds, peroxide moieties, etc.) is probably NMR spectroscopy (1H- and 13C- both in solution and in the solid state) [13,14]. However, it requires expensive instruments not easily available for routine measurements and skilled technicians able to derive information from the complex spectra. IR spectroscopy has been widely used to characterize PE samples and, in particular, to detect the presence of various oxidation products that are formed as a consequence of degradation reactions in the presence of oxygen [3,5,7,8]. In principle, a quantitative evaluation of peroxide and hydroperoxide groups could be used as an index of the tendency of HDPE to undergo degradation in service; however, the relatively low rate of formation of these groups in HDPE [1,5,7] and the common presence of additives and carbon black in commercial PE-pipes can reduce the sensitivity of this technique and make difficult to derive information from such measurements. In order to characterize the samples listed in Table 1 with respect to the content of chemical moieties that can behave like degradation promoters, ATR-FT-IR spectra have been recorded. No significant differences appeared in the spectra and in particular no evidence of specific bands in the wave-number regions 1600–1800, 800–1100 and 3200–3700 cm1, typical of carbonyl and carboxyl, vinyl and vinylidene, and hydroxyl and hydroperoxide groups,
respectively [5,8], were observed even in PE100UP, which was expected to contain recycled PE. TGA has also been proposed as a possible technique to investigate the HDPE degradation [15]; as the rate of weight loss is expected to be related to the rate of formation of low molecular weight fragments from degradation reactions. For all the samples listed in Table 1, TGA curves were recorded at 30 C/min up to 800 C, under a stream of either N2 or air. Under air, no residual weight was present at 800 C; therefore, the residual weight remaining at 800 C under N2 can be reasonably attributed to the carbon black (initially contained or formed by pyrolysis of HDPE). The residual weight percentage at 800 C under N2 is reported in Table 1. As it can be seen, the residual weight percentages are the same for virgin PE samples (PE100VG, PE100VP, PE80VG) and significantly higher for sample PE100UP. Even though the real origin and composition of sample PE100UP are unknown, this result supports the suspicion that it contains recycled PE, as it is reasonable to think that a higher carbon black content was added to improve the initial mechanical properties of the HDPE pipes. Of course, this evidence cannot be considered conclusive with respect to the qualification of HDPE. Fig. 1 shows TGA curves recorded under N2 and air. The temperature at which there is the maximum rate of weight loss can be taken as an indication of the resistance to degradation. As expected, the degradation occurs faster under air than under nitrogen, the maximum weight-loss rate occurring at lower temperatures for both PE100VG and PE100UP. The maximum weight loss rate for PE100UP occurs at lower temperature with respect to PE100VG both under air and nitrogen, suggesting a lower resistance to degradation for PE100UP. This last result supports the hypothesis that sample PE100UP contains recycled PE and suggests that TGA is able to evidence a faster degradation rate for sample PE100UP with respect to virgin PE100VG. However, it has also to be considered that the shape and position of TGA curves can depend on the initial molecular weight of the samples, and a significant contribution to the difference between the temperature of maximum weight
Table 2 Operating conditions for thermo-mechanical-oxidative ageing in the internal mixer at 190 C. Method Feeding Rotor speed Mixing PE100VG PE100VP PE100UP weighta (g) (rpm) time (h) A B C D Eb
54 45 45 35 45
30 30 30 30 10–100b
4 4 1 1 1
X X X X X
X X
X X X X X
a
IM chamber is completely filled with 54 g of PE; as the IM chamber was open to air, the amount of oxygen within the mixing chamber was dependent on the sample weight feeded. b Various rotor-speeds, from 10 to 100 rpm, have been used.
Fig. 1. TGA curves recorded at 30 C/min heating rate for samples PE100VG, PE80VG and PE100UP, either under nitrogen (N2) or under air.
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loss rate of samples PE100UP and PE100VG may derive from the significantly lower molecular weight of sample PE100UP rather than from the different degradation rate. Indeed, if we compare the TGA curves of sample PE100VG and PE80VG, both from virgin samples but with different initial MFIs, it is evident that there is a significant shifting to lower temperature for sample PE80VG with a higher MFI. Furthermore, the curve obtained for the virgin PE80VG is close to that of PE100UP, and it can be concluded that it is not easy to separate the effects of degradation rate from those of molecular weight. As a consequence, TGA seems not suitable for the aim of this work. DSC has also been used to investigate HDPE degradation [1,2,14,16]. In principle, changes in the chemical structure occurring upon degradation, such as changes in molecular weight and/or formation of branching, could induce changes in crystallization and melting behaviour that could be recorded by DSC. For all samples, DSC curves were recorded either under cooling (from the molten state at different cooling rates), under isothermal conditions (121 C) and under heating at 20 C/min. The most relevant data are summarized in Table 3. Just small differences appear between different samples and they cannot be easily related to the degradation rate behaviour expected according to the previous thermo-mechanical history of the samples. Finally, to complete the characterization of the as received samples, MFI was measured according to ISO 1133:1997. Extruded samples were collected from the MFI apparatus every 10 min over 1 h. MFI data are reported vs time in Fig. 2. As it appears, there is a slight increase of MFI for samples taken at increasing times, within 1 h, suggesting that degradation occurs during the measurement. Initial values of MFI were therefore obtained by extrapolation to zero-time and are reported in Table 1. As expected, the MFI values are very similar for samples PE100VG and PE100VP, whereas MFI is significantly higher for PE100UP. 3.2. Degradation upon conventional ageing One of the most used methods to investigate durability of polyolefins is to perform accelerated embrittlement tests on solid samples aged in an oven at high temperature (typically just below the melting temperature). Extrapolation to low temperature is then used to get information about durability during service life. However, this ageing method requires several weeks or months to give a response and it is not suitable for the aim of this work. It is expected that the higher the ageing temperature the faster is the degradation rate, therefore, in order to achieve information about the degradation rate in a relatively short Table 3 DSC data of as received and IM aged samples recorded under crystallization at 20 C/min. Sample code
PE100VG PE100VP PE100UP
As received (unaged)
Aged
Tc,peak ( C)
DHc (J/g)
Tc,peak ( C)
DHc (J/g)
106.1 102.2 104.9
137.4 142.5 133.8
109.2 103.5 104.0
135.6 147.2 132.0
99
Fig. 2. MFI of PE100VG pellets and pipes and PE100UP pipe at 190 C.
time, the samples listed in Table 1 were submitted to conventional accelerated ageing at 190 C, in the molten state, within the MFI chamber (for times up to 16 h). Aged samples were then submitted to FT-IR, TGA, DSC and MFI characterization in order to investigate the changes induced by ageing. Neither FT-IR nor TGA and DSC gave significant changes after ageing. On the contrary, some significant differences were observed in MFI measurements. Fig. 2 shows the MFI data recorded up to 260 min for samples heated under shear (from 0 to 70 min and from 220 to 280 min) or under static heating (from 70 to 220 min). As it can be seen, MFI slightly increases under shear, whereas a slight decrease of MFI is observed after static heating. These results are in agreement with other ones previously reported in the literature [3], and suggest that when mechanical stresses are applied the degradation mechanism changes with respect to that occurring during static heating. If we consider that both chain scission and branching reactions can occur concurrently at the same time, MFI results suggest that chain scission prevails over branching/crosslinking under shear, whereas the contrary seems to happen under static heating. In order to understand if the MFI decrease can be used to investigate the durability characteristics of different samples, MFI tests were performed on samples submitted to static heating for various times (4, 8 and 16 h); the results are reported in Table 4. Changes in MFI are very limited (values are almost unchanged) for samples PE100VG and PE100VP, suggesting either a low degradation rate or compensation of opposite Table 4 MFI values measured after different heating times in the MFI chamber at 190 C. Ageing time (h)
PE100VG MFI (g/100 )
PE100VP MFI (g/100 )
PE100UP MFI (g/100 )
0 4 8 16
0.18 0.176 0.20 0.20
0.20 0.18 0.18 0.20
0.65 0.52 0.51 0.56
0.02 0.004 0.01 0.02
0.02 0.02 0.01 0.01
0.01 0.02 0.03 0.04
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effects such as those that would be originated by chain scission and branching. More significant changes were obtained for sample PE100UP; a relatively fast decrease of MFI after 4 h of ageing, is followed by a slight increase over longer times. These results suggest different behaviour towards degradation of samples PE100VG and PE100UP, but the effect of ageing is too limited and therefore unsuitable for the development of a ranking criterion for unknown HDPE samples. 3.3. Degradation upon ageing in internal mixer Due to the limited effects observed upon ageing under static heating in the MFI chamber at 190 C, it was decided to move to stronger ageing procedures, able to induce more extended changes in the samples in a relatively short time. According to the literature [3] and in agreement with the above reported MFI results, degradation phenomena are expected to be faster when PE samples are submitted to melt mixing in the presence of oxygen and under mechanical stresses. In order to have all these factors operating at the same time, samples were melt blended in an internal mixer (IM) at 190 C for various times, using different rotor speeds and with the IM chamber either completely (54 g) or partially filled (45 and 35 g) with samples (see Table 2). After melt mixing, the recovered samples were ground and investigated by FT-IR, DSC, and MFI. Again, no significant changes were observed in FT-IR spectra and in DSC curves, while relevant changes were observed in MFI. Fig. 3 shows DSC traces before and after
ageing (according to method C), as an example. It is interesting to note that small changes were found for PE100VG and PE100VP, whereas almost identical curves were found for sample PE100UP, which, on the contrary, was expected to undergo a more extended degradation and, therefore, to show the strongest changes. Results of MFI measurements are reported in Table 5. As can be seen, limited changes were observed after ageing at 30 rpm for 4 h, when the IM chamber was completely filled (ageing method A). On the contrary, very significant changes were observed when the amount of sample in the IM chamber was reduced to 45 g (ageing method B). Significant changes were also observed with the same amount of sample (45 g) for a shorter time (1 h, ageing method C). Instead, limited changes were observed by further reducing the amount of sample to 35 g (ageing method D). It is evident that the amount of sample charged in the IM chamber plays a relevant role. This is not surprising, as a reduced amount of sample in the IM chamber is expected to affect both the volume of air within the IM chamber (the lower is the polymer charged the higher is the volume of air in the IM chamber and, therefore, the amount of oxygen that is in contact with the polymer during melt mixing), and the nature and intensity of the stresses acting on the molten polymer. When the IM chamber is completely filled, the mechanical stresses are expected to be the highest, but the amount of oxygen that is in contact with the polymer is limited. On the other hand, when a too small amount of sample is charged in the IM chamber (35 g) the friction of the molten polymer with the chamber walls is reduced and the polymer is expected to
Fig. 3. DSC crystallization curves recorded at 20 /min after melting at 200 C for samples as received and after ageing by method C (sample/a).
T. Zanasi et al. / Polymer Testing 28 (2009) 96–102
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Table 5 MFI values (and percent change with respect to unaged samples) of PE100VG, PE100VP and PE100UP samples after different thermo-mechanical ageing treatments in the internal mixer chamber at 190 C and 30 rpm. Ageing in the IM chamber Method A B C D
PE100VG MFI (g/100 )
DMFI (%)
MFI (g/100 )
DMFI (%)
MFI (g/100 )
DMFI (%)
0.18 0.26 1.05 0.27 0.24
–
0.20 0.02
–
0.65 0.83 3.6 1.09 0.99
–
54 45 45 35
0 4 4 1 1
0.02 0.02 0.04 0.01 0.01
4. Discussion and conclusions Considering that premature failure of PE pipes for water and gas pipe applications should be avoided as it causes consumer’s discomfort and requires costly maintenance operations, it is obvious that a method able to predict durability would be highly appreciated. Durability of PE pipes is expected to be strongly related to the rate of degradation reactions occurring during service life and an accelerated testing method able to predict a too fast degradation in service would be highly desirable. It should be reliable, fast and simple, so that it can be carried out in small/medium size factories where PE pipes are produced or used, by people not particularly scientifically skilled and using relatively inexpensive apparatuses. Accelerated ageing at high temperature is a common practice used to increase the rate of degradation reactions in order to reduce the experimental time required to obtain relevant changes in some properties and information about the useful service life. However, the commonly used ageing methods (static heating of solids and repeated extrusions) Table 6 Effect of rotor speed on MFI for samples submitted to melt mixing at 190 C for 1 h, in a partially filled (45 g) IM chamber. Sample code
Rotor speed (rpm)
MFI (g/10’)
PE100VG
10 30 50 100 10 30 50 100 10 30 50 100
0.19 0.26 0.72 2.92 – 0.30 0.78 3.13 – 1.10 – 2.40
PE100UP
PE100UP
Mixing time (h)
stick to the rotors and to move integrally with them, with limited or no stresses (both elongational and shear stresses) acting on the polymer melt. The effect of rotor speed over 1 h mixing has also been investigated in a partially filled IM chamber, as appears from the data of Table 6. An increase in the rotor speed from 10 to 100 rpm leads to a progressive increase in MFI for all samples, however, the MFI increase registered for sample PE100UP is lower than for PE100VG and PE100VP, suggesting a different degradation mechanism.
PE100VP
PE100VP
Feeding weight (g)
9 349 17 4
0.30 0.02
36
0.01 0.05 0.2 0.06 0.05
22 432 60 45
require either too long times or are almost ineffective, in particular when HDPE contains stabilizers [7]. There is much evidence in the literature suggesting that the relative rate of the various degradation reactions may be different under different ageing conditions and, in particular, it has been reported that a different reaction mechanism and synergistic effects may derive from the simultaneous combination of different factors such as temperature, oxygen concentration and mechanical stresses [3]. For example, when HDPE is subjected to a shear stress, the degradation is mainly due to primary reactions, i.e. chain scission of polymer chains rather than reactions involving oxidation products accumulated in the system. Nevertheless, previous studies on HDPE ageing have mainly dealt with factors able to accelerate degradation reactions separately, typically by submitting HDPE samples either to repeated extrusions or to static oxidative oven ageing, and only seldom has a combination of these factors been used [3]. On the other hand, the results of this study have shown that ageing in IM under a suitable combination of temperature, oxygen concentration, time and rotor speed can be effective, leading to a strong degradation of HDPE sample in a relatively short time. To demonstrate the effects of accelerated ageing of HDPE, various techniques are available. Embrittlement of solid specimens, changes in FT-IR and NMR spectra, different evolution rates of low molecular weight by products on heating (TGA, GC–MS), changes in crystallization rate and in the extent of crystallization and in rheological properties are the expected results of ageing. All these techniques have been extensively used to investigate PE degradation. Embrittlement tests, one of the most used methods to derive information about durability, require ageing times from several weeks to few months, too long a time for many purposes. On the other hand, spectroscopic methods (NMR and FT-IR), the more appropriated techniques for scientific investigation of the degradation mechanism, require costly instruments, skilled personal and time consuming calibrations, and the results are not easily related to durability. The presence of additives and carbon black in commercial samples is a further complication that requires long and expensive operations in order to obtain samples suitable to record clear spectra. As reported in the literature, changes in the spectra of HDPE are difficult to observe even after very long ageing times [1,5]. Spectroscopic techniques (FT-IR) and thermal characterization (DSC and TGA) have been found unsuitable for the purpose
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of this article when applied to commercial pipe-grade HDPE. On the contrary, MFI proved to be effective under suitable ageing conditions (method C in particular) and it was possible to differentiate the degradation behaviour of virgin HDPE granules (PE100VG) from good-quality HDPE pipes (PE100VP) and from poor-quality pipes (PE100UP). Indeed, melt flow is strongly affected by the molecular structure (molecular weight, MWD, presence of branching) and, therefore, it is not surprising that MFI measurements have been found the most sensitive to reveal the effects of ageing among the various techniques used. The combined effect of temperature, stresses, oxygen and residence time, such as method C, seems to be a very effective ageing method able to give interesting results for HDPE qualification when combined with MFI measurements. Too long ageing times lead to too strong increases of MFI, the effect of degradation are so high that the discrimination capability between samples PE100VG and PE100UP is reduced. An increase of mechanical stress by increasing rotor speed (compare tests in Tables 5 and 6) has similar effects; a too high rotor speed leads to too rapid degradation that reduces the difference in MFI changes for different samples and makes it difficult to rank HDPE samples for their resistance to oxidation. A too high increase of the volume of air in the IM chamber is more than counterbalanced by a too strong decrease of mechanical stress, with a significant reduction of the overall effect of ageing on MFI (compare tests C and D). On the other hand, a too small amount of air in the IM chamber reduces the effects of ageing, presumably because of the smaller amount of oxygen able to come in contact with PE chains (compare sample A and B). By comparing the results obtained for samples PE100VG, PE100VP and PE100UP after ageing with method C, we can observe that the MFI increase is 17%, for PE100VG (virgin HDPE), 30% for PE100VP (pipes from virgin HDPE) and 60% for PE100UP (granules obtained from a pipe that probably was prepared by using also recycled PE). Thus, ageing method C seems able to rank samples for their resistance to degradation, as expected according to their previous thermo-mechanical history. In particular, it is interesting to note that Method C is very sensitive, being able to detect even the effects of an additional extrusion step on the degradability of virgin HDPE (compare MFI values for PE100VG with those for PE100VP). Even though more tests on a larger number of PE samples are necessary to definitely conclude that ageing
method C is a general one applicable to rank PE pipes for their durability (various PE sample are under investigation), we can conclude that it is a simple, easy and fast method able to demonstrate differences in the degradation behaviour of commercial HDPE samples. Finally, it has to be noted that both IM and MFI are apparatuses that can be easily managed by people not particularly scientifically skilled, and that these apparatuses are frequently present in small/medium size factories.
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