AC Dielectric Strength of Polyethylene Terephthalate Under Thermal Aging

Polyethylene terephtalate (PET) is widely employed in industry [1] because of its good mechanical properties, low water absorption and resistance to inorganic chemicals [2]. Yang et al. [3] presented the properties of PET: glass transition temperature T_g=80℃, melting point T_m=258℃, density r=1.40 g/cm³, relative permittivity at 50 Hz e_r=3.1 and hygroscopicity h=0.4% Despite of its advantages, PET degrades under thermal aging.

Panowicz et al. [2] examined the properties of PET after thermo-oxidative aging. Sheets of the material were aged in oven heated at 140℃ in such a way that air could freely flow around the samples. Specimens were withdrawn from the oven after 21, 35 and 56 days. The study shows that the quantity of the crystalline phase raises by about 8%. The glass transition and melt temperatures heighten with aging time. The tests point a raise of Young’s modulus and a shortening of elongation at rupture for the temperature range between 25 and 75℃. A change in fracture character of the insulation from ductile to brittle was noticed.

Samperi et al. [4] considered thermal aging of PET at the temperature between 270 and 370℃. The authors reported that cyclic oligomers are formed but decompose at higher temperature. Whereas the formation of anhydride containing-oligomers is well apparent. Acetaldehyde was detected in aged specimens.

McNeill and Bounekhel [5] conducted thermal stabilities of PET by thermogravimetry and thermal volatilisation analysis. The authors exhibited that the initial scission happens at the ester linkage to give terminal carboxyl and vinyl structures in the chains. For all the temperatures at which volatile products are released, carbone monoxide and carbone dioxide are formed.

Bárány et al. [6] viewed the influence of thermal aging on sheets of PET with about 0.3 mm thickness. The aging was realized, at just above the glass transition temperature, until 264 h. The researchers displayed that the aging causes an increase of Yield stress and an embrittlement of the samples.

Chipara et al. [7] reported the thermooxidative degradation of PET at 125 and 150℃. The authors pointed a decrease of elongation at break and tensile strength versus aging time. At 150℃, a transition from crosslinking to chain scissions was noted after 500 h. It was noticed that the glass transition temperature varies in function of aging time at 125℃. The researchers closed that the thermooxidative decomposition is governed by a first - order process.

For the use of an insulating material in a device, it is necessary to study the effect of thermal aging on its properties. It is useful to determine the temperature index which is the temperature corresponding to a lifetime of 20000 h. The goal of the present work is to investigate the evolution of dielectric strength of PET under thermal aging. The PET was characterised by Fourier Transform Infrared (FTIR) and Thermogravimetric Analysis (TGA).

3.1 Change of dielectric strength against aging time

Figure 1 illustrates the change of dielectric strength (E_b) with respect to aging time at different temperatures. The change can be depicted as below:

• At 130℃, E_b increases rapidly from 52.50 kV/mm to 63.83 kV/mm, lowers to 57.46 kV/mm and rises until 61.82 kV/mm after 1500 h. Next E_b lessens slowly to 60.84 kV/mm for 3000 h. Then E_b increases a little to 62.06 kV/mm, diminishes slightly until 61.89 kV/mm and grows to 62.55 kV/mm for 4500 h. Later E_b reduces to 58.23 kV/mm, raises to 62.95 kV/mm and decays to 60.77 kV/mm then enlarges somewhat to 61.21 kV/mm after 8500 h. The maximum modification is 21.58%.
• At 140℃, E_b grows quickly from 52.50 kV/mm to 64.44 kV/mm and decays until 58.79 kV/mm for 2500 h. After E_b heightens to 63.60 kV/mm, falls to 47.97 kV/mm and raises quickly until 63.26 kV/mm after 4000 h. After E_b lowers to 58.41 kV/mm, enhances to 64.42 kV/mm and weakens to 63.71 kV/mm then it rises a rather to 64.78 kV/mm for 6000 h. The maximum changing is 23.39%.
• At 150℃, E_b lowers slowly from 52.50 kV/mm to 50.00 kV/mm, raises quickly until 68.16 kV/mm and shortens again to 63.26 kV/mm after 4000 h. The maximum change is 29.83%.
• At 160℃, at first E_b is practically constant; its value is about 52.50 kV/mm. Then it increases to 56.41 kV/mm and lowers until 54.23 kV/mm for 1500 h. Beyond this time, E_b raises to 58.17 kV/mm and drops to 51.13 kV/mm after 3000 h. The maximum variation is 10.82%.
• The mean, standard deviation and error bars of E_b were calculated. The results are presented in the Tables 1-2.

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Figure 1. Dielectric strength versus aging time a) at 130℃; b) at 140℃; c) at 150℃; d) at 160℃

Table 1(a). Mean and error bars of E_b before and after aging at 130℃

Table 1(b). Mean and error bars of E_b after aging at 140℃

Table 1(c). Mean and error bars of E_b after aging at 150℃

Table 1(d). Mean and error bars of E_b after aging at 160℃

Table 2(a). Standard deviation of E_b before and after aging at 130℃

Table 2(b). Standard deviation of E_b after aging at 140℃

Table 2(c). Standard deviation of E_b after aging at 150℃

Table 2(d). Standard deviation of E_b after aging at 160℃

3.2 Modification in sample colour

Figure 2 shows the insulation before and after aging. As it can be seen, the PET color changed after aging.

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(a)                                                                             (b)

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(c)                                                                            (d)

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(e)

Figure 2. Photographs of the polymer: a) Before aging; b) After 8500 h at 130℃; c) After 6000 h at 140℃; d) After 4000 h at 150℃; e) After 3000 h at 160℃

3.3 FTIR analysis

3.3.1 Before aging

Table 3 presents the significant infrared (IR) absorbance bands of PET ascribed to the vibrations as yielded by Chércoles Asensio et al. [9].

Table 3. Infrared absorbance bands

Figure 3(a) shows the IR spectrum before aging. This spectrum can be summarized as follows:

- It was detected an absorbance band at 1710 cm^-1 ascribed to stretching vibration of C=O, characteristic of ester.

- The absorbance bands, noticed at 1408 and 1344 cm^-1, are related to symmetric and asymmetric bending vibration in-plane C−H and rocking bending of C – H − CH₂−.

- The absorbance band, shown at 1238 cm^-1, is allocated to the stretching vibration of C− C (O) − O.

- The absorbance band, arising at 1097 cm^-1, is due to the stretching vibration of – O – C −.

- Two absorbance bands are identified at 1017 and 962 cm^-1 corresponding to the bending vibration in-plane of =C − H.

- The bending vibration out-of-plane of =C − H occurs at 872 cm^-1.

- The absorbance band, emerging at 717 cm^-1, matches to the wagging bending vibration of =C − H.

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Figure 3(a). IR spectrum before aging

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Figure 3(b). IR spectrum after 4000 h at 130℃

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Figure 3(c). IR spectrum after 8500 h at 130℃

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Figure 3(d). IR spectrum after 4000 h at 140℃

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Figure 3(e). IR spectrum after 6000 h at 140℃

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Figure 3(f). IR spectrum after 2000 h at 150℃

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Figure 3(g). IR spectrum after 4000 h at 150℃

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Figure 3(h). IR spectrum after 1000 h at 160℃

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Figure 3(i). IR spectrum after 3000 h at 160℃

3.3.2 After aging

Figures 3(b)-(i) represent the IR spectra after aging. As one can notice, the peak intensities of absorbance bands at 1710, 1238, 1097, 1017, 872 and 717 cm^-1, decreased. Furthermore, we remark that the absorbance bands at 1408, 1344 and 962 cm^-1 vanished.

3.4 TGA thermograms

As one can observe, the TGA thermograms have the same form. Between 25℃ and 300℃, a mass loss of about 1% was remarked before and after aging. This mass loss is assigned to the evaporation of water and volatile solvents.

3.4.1 Before aging

Figure 4(a) shows the thermogravimetric curve before aging. At the beginning, the mass loss begins at around 402℃, expedites and reaches 94% at 457℃. Next, the mass slackens and it remains a residue of 5.3% at 797℃. The temperature matching to 50% mass loss is 432℃.

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Figure 4(a). TGA thermogram before aging

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Figure 4(b). TGA thermogram after 4000 h at 130℃

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Figure 4(c). TGA thermogram after 8500 h at 130℃

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Figure 4(d). TGA thermogram after 4000 h at 140℃

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Figure 4(e). TGA thermogram after 6000 h at 140℃

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Figure 4(f). TGA thermogram after 2000 h at 150℃

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Figure 4(g). TGA thermogram after 4000 h at 150℃

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Figure 4(h). TGA thermogram after 1000 h at 160℃

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Figure 4(i). TGA thermogram after 3000 h at 160℃

3.4.2 After aging

Figures 4(b)-(i) give the TGA thermograms after aging.

3.5 Discussion

1. The aging heightens the thermal agitation producing a progressive lowering in the viscosity. Thereby the molecular bonds decay and the free volume grows. Consequently, the mean free path of charge carriers increases causing the raise of their mobility. This phenomenon explains the shortening of E_b. In the contrary, the enlargement of dielectric strength is ascribed to the rearrangement in the molecular chains inducing a shortening in the mean free path and the mobility of charge carriers. This phenomenon was reported elsewhere [10]. Xing et al. [11] pointed out the growth of the free volume in polypropylene with respect to temperature. The free volume increases from 1.23 ×10^-9m³ at 25°C to 1.56 × 10^-9 m³ for 90°C.
2. Statistical study indicates that the mean of E_b is ranged from 47.97 to 68.16 kV/mm. The error bars are situated between 0.959 and 1.363 kV/mm. The standard deviation varies between 1.22 and 8.22 kV/mm.
3. Figure 1(b) presents a maximum of 64.44 kV/mm, after 1000 h, which may be due to the crystallization. The curve shows also a minimum of 47.97 kV/mm, after 4500 h, which may be assigned to the chain scission.
4. For all temperatures, the sample colour changed after aging. Moreover, the dielectric material became brittle after aging. During the heating, we remarked a decrease in the adhesion of the film to the sheets.
5. Dielectric strength is affected by the presence of defects which can exist within the polymer during the making or created by the aging. Katsuta et al. [12] indicated that breakdown strength of XLPE cables decreases versus void size. Hagen and Ildstad [13] established that the addition of conducting iron and copper particles to XLPE insulation shortens AC dielectric strength from 107 kV/mm to 70 kV/mm and 35 kV/mm, for spherically and irregularly shaped particles respectively. For the samples holding glass particles, the dielectric strength diminishes to 55 kV/mm independently to the form of particles. Chen and Davies [14] examined the behaviour of low-density polyethylene under DC electric stress. The authors reported that, after aging, the samples pointed out oxidation degradation around the defects due to the enhancement of electric field.
6. The meaning infrared absorbance bands, listed in Table 3, were reported elsewhere [15-18]. After aging, a diminution in the intensities of absorbance band peaks was noted. In addition, an extinction of some absorbance bands was observed. The FTIR peak loss may be attributed to the chain scission and the decrease of molar mass as reported by Oreski et al. [19] This phenomenon leads to the increase of the dielectric strength. For a fixed temperature, the residue trends to increase versus aging time. Besides, E_b tends to raise against heating time. The rate of reactions can be given by [20]:

$r=\frac{\partial \propto}{\partial t}=A \exp \left(-\frac{E}{R T}\right)(1-\alpha)^n$                             (1)

$\alpha=\frac{\omega_0-\omega}{\omega_0-\omega_f}$                       (2)

where, a is a conversion, t is the time, A is the pre-exponential factor, E is the activation energy, R is universal gas constant, T is temperature, and w₀, w, w_f are the weight of the sample at initial time (t=0), time t and at the end of the TGA experiment, respectively, and n is the reaction order.  

The dielectric strength depends on the molar mass and the chain scission. Thence it is linked to the thermal degradation kinetics.

7. The TGA thermograms exhibit that the decomposition of the polymer takes place depending on one stage: the process is governed by a first-order chemical reaction. This phenomenon was reported elsewhere [15, 21]. The graphs display modifications of onset temperature and residue. The beginning temperature varies between 394℃ and 404℃. The residue alters from 5.3% to 17.83%.
8. PET degradation involves ester hydrolysis and thermo-oxidation. Holland and Hay [22] related thermal degradation of PET. The authors showed that the presence of diethylene glycol and isophthalate units has a significant influence on the process, increasing the chain flexibility and creating more favourable bond angles. The thermal degradation causes the formation of non-volatile residue contained almost exclusively of interconnected aromatic rings.
9. Complementary tests, at longer times of heating, are useful to determine thermal endurance curve of PET. The lifetime will be established for a 50% decrease of dielectric strength for all the temperatures, which determines the temperature index.