Accelerated aging and characterization of HDPE pin type insulators (15 kV)

Abstract

The study of the behavior of polymeric material used for insulators in the electrical system is extremely important in order to evaluate their lifetime as well as their performance when exposed to different environmental conditions. In the present work, the behavior of high-density polyethylene (HDPE) pin type insulators (15 kV), under accelerated aging conditions, was studied. Samples were exposed to aging, for 200 h, 1000 h, and 2000 h, in accelerated weathering chambers, according to two different methods. In Method 1, the parameters were established based on natural aging in location conditions, and in Method 2, ASTM G155 standard parameters were used. All samples were characterized by rheometry, Fourier-transform infrared spectroscopy (FTIR), and differential scanning calorimetry (DSC). The accelerated aging conditions, used in Methods 1 and 2, affected the aging of HDPE pin type insulators samples differently, the changes being more intense in Method 1.

Keywords

Polymeric insulator high-density polyethylene accelerated aging FTIR differential scanning calorimetry

Introduction

Electrical insulation is one of the most important parts of all electrical components, and its quality determines the reliability of several electric equipment used in electric energy generation, transmission, and distribution, such as cables, capacitors, insulators, transformers, generators, and motors. Every electric insulation system is designed to support the different stresses, which equipment and components are exposed to, during their lifetime.^1,2

The study of the behavior of the polymeric material used for manufacturing electric system insulators is important in order to evaluate the lifetime of such products, as well as their performance when exposed to different environmental conditions.³ Despite the advantages of the use of polymeric insulators, there are many concerns related to them, such as the diversity of polymeric materials and additives and the reliable prediction of the lifetime of these insulators, among others.^4,5

Aging studies are extremely important for the development of more resistant materials, as well as additives, for increasing the lifetime of polymers.⁶ The use of accelerated aging methods makes possible the prediction of material behavior and their lifetime, at short periods of exposure.⁷

For large thickness products, the degradation is, usually, limited to a superficial layer, due to the difficulty for the diffusion of oxygen or for the penetration of UV radiation.⁸ The loss of antioxidant protection of polymers is a subject largely reported in scientific and technical communications.^9–17

The lifetime polymeric material evaluation used for insulators in the electrical system as well as their performance when exposed to different environmental conditions is extremely important. So, in the present work, the behavior of high-density polyethylene (HDPE) pin type insulators (15 kV), using accelerated weathering chamber, at different conditions, was studied. Temperature, humidity, and irradiance were the parameters used for accelerated aging conditions. Samples were exposed to aging in accelerated weathering chambers, according to two different methods. In Method 1, the parameters were established based on natural aging in local conditions, and in Method 2, ASTM G155 standard parameters were used. The characterization of the insulators, before and after aging, was carried out by differential scanning calorimetry (DSC), FTIR, and rheometric tests. The obtained results for Methods 1 and 2 show that the aging of HDPE pin type insulator samples was differently affected. It is believed that the same behavior should be observed in other polymers used in the electrical system.

Materials and methods

Samples prepared according to ASTM D638 and HPDE pin type insulators (15 kV), as purchased, were used. Accelerated aging was carried out using two accelerated weathering chambers, a ZUNDAR ZTS061L (metal iodide lamp), identified in this study as C1, and an ATLAS CI4000 (xenon lamp), identified as C2. The samples were exposed to accelerated aging, in C1 chamber, for 200 h, 1000 h, and 2000 h, in 200 h cycles, under 748 W m⁻² of average irradiance (280 nm–3000 nm spectrum). These cycles were carried out as follows: 133 h at 45°C, relative humidity between 50% and 70%, followed by 67 h at 0°C, without humidity insertion. This procedure was identified as Method 1, and it was conducted with integral insulators as samples. The conditions used in this method were an attempt to simulate the natural ones, concerning the main stresses to which the insulators are exposed to, that is, average irradiation, temperature, and humidity. The temperature/humidity simulation considered summer and winter cycles. For C2 chamber, ASTM G155¹⁸ standard parameters were used. The samples were exposed in this chamber for 200 h, 1000 h, and 2000 h, divided into 100 cycles of 102 min each, at 63°C, under UV light, followed by 18 min, under UV light and water sprinkling, under 0.35 W m⁻² (340 nm) of average irradiance, for each 200 h period. In such method, the samples were prepared into dumbbell-shaped specimens, according to ASTM D638.¹⁹ This procedure was identified as Method 2. The evaluated samples were identified according to the aging time they were exposed to, as presented in Table 1.

Table 1.

Identification of the evaluated samples and their respective collection point.

Method	Aging time (h)				Sample collection point
Method	0	200	1000	2000	Sample collection point
1	A0	A1	A2	A3	1st insulator skirt, below the top
2	B0	B1	B2	B3	Specimen face exposed to radiation

FTIR analysis

A Thermo Fisher Scientific, model Nicolet 6700, FTIR spectrometer, ATR mode, with 64 scans and 4 cm⁻¹ resolution, was used for obtaining molecular absorption spectra in the infrared region, ranging from 4000 cm⁻¹ to 650 cm⁻¹. With the aid of this technique, the Carbonyl Index (CI), for the aged samples, was determined, based on equation (1)²⁰

CI = \frac{I [spectra region between 1750 {cm}^{- 1} and 1725 {cm}^{- 1}, attributed to carbonyl (C = O)]}{I (reference: CH 2 group absorption band between 1472 {cm}^{- 1} and 1462 {cm}^{- 1})}

(1)

where I is the intensity of the absorption band (in absorbance).

Differential scanning calorimetry analysis

A Shimadzu, DSC-60 model, with aluminum sampler, was used. Crystalline melting temperature (T_m) and degree of crystallinity were determined according to ASTM D3418.²¹ The Onset Oxidation Temperature (OOT) test was conducted according to ASTM E2009,²² using synthetic air as carrier gas.

Rheometric analysis

Rheometric tests were performed on a rotational rheometer, AR-G2 model (TA Instruments), using plate–plate geometry, at 190°C. The linear viscoelastic region was determined from storage (G') and loss (G'') moduli plots as a function of strain (%), in a strain sweep test. The frequency sweep test was performed to determine the variation of the weight average molecular weight (M_w) of the samples as a function of aging. The software Rheology Advantage Data Analysis version 5.7.0 was used. The frequency was varied from 0.8 to 400 rad s⁻¹, and, as a control variable (% strain), the value of 2.5% was used, which is within the linear viscoelastic region.

Results and discussion

FTIR analysis

The FTIR spectrum of HDPE is characterized by three intense methylene (CH₂) group bands, which appear as doublets at 730 cm⁻¹ and 718 cm⁻¹, 1472 cm⁻¹ and 1460 cm⁻¹, and 2917 cm⁻¹ and 2849 cm⁻¹. Other characteristic peaks include vinyl group peaks (−C = C) in the range of 1000 cm⁻¹–900 cm⁻¹, carbonyl group peaks (−C = O) in the range of 1800 cm⁻¹–1500 cm⁻¹, and hydroxyl group peaks (−OH) in the range of 3600 cm⁻¹ to 3100 cm⁻¹.²³ A comparison between the FTIR spectra of the non-aged sample (A0 and B0) and the samples aged for 2000 h, by Method 1 (A3) and by Method 2 (B3), is shown in Figure 1(a). Concerning the FTIR spectra of these samples, characteristic bands of polyethylene were identified, attributed to CH₂. High intensity bands at 2917 cm⁻¹ (asymmetric stretching), at 2849 cm⁻¹ (symmetric stretching), between 1472 cm⁻¹ and 1462 cm⁻¹ (bending angular strain), and a medium intensity band between 730 cm⁻¹ and 718 cm⁻¹ (rocking). No significant differences were observed, in the FTIR spectra, for samples, aged for 200 h and for 1000 h, by both methods.

Figure 1.

(a) FTIR spectra for samples not aged and aged by Methods 1 and 2 to 2000 h. (b) In detail the same scaled-down spectra for absorption.

It is possible to observe the emergence and/or intensity increase of some bands, indicated in Figure 1(b). The bands around 3348 cm⁻¹ and 1736 cm⁻¹ are related to the degradation of polyethylene, due to the formation of hydroperoxides and carbonyl, respectively.²³ The increase of the band intensity, at 3348 cm⁻¹, as shown in the FTIR spectra of the samples aged for 2000 h, in comparison with the spectra for the non-aged ones, might be probably related to the polymer loss of hydrophobicity, along aging and according to the aging method. The other bands, indicated in Figure 1, might well be related to a possible presence of additives at the samples surface, that is, $N H_{2}^{+}$ (a secondary amine), that absorbs close to 1600 cm⁻¹, SiO (silicate), that absorbs between 1100 cm⁻¹ and 1000 cm⁻¹, and the functional group of carbonate ion ( $C O_{3}^{−2}$ ), with bands close to 1415 cm⁻¹ and 872 cm⁻¹.^24,25 These groups can be found in sterically hindered amine-based additives (HALS), pretty much used as photo-stabilizer and in silica, that increase the resistance of the polymer to electrical tracking. The possible presence of additives on the surface of the aged samples might be related to the migration of these additives to the surface, being more intense than on the non-aged sample, as can be interpreted by the FTIR results, presented in Figure 1. For the validation of this hypothesis, it would be necessary to know the chemical composition of the HDPE insulator, determining the concentration of each additive, before and after aging, which was not carried out in the present work.

Haider and Karlsson¹⁰ and Du and collaborators¹⁶ studied the loss and migration of additives in LDPE films and in epoxy resin surface, respectively. The FTIR results of these studies indicated the loss and migration of these additives during aging.

According to Haider and Karlsson,²⁶ the loss rate of the additive depends on its compatibility with the polymer, and it is controlled by its mobility, ease of extraction, and solubility. The additive migration process, during the polymer lifetime, results in changes in polymer properties. Yang and collaborators²⁷ verified that small volatile molecules can migrate to the surface during polymer aging, and these changes can be detected and used to represent the aging condition of the polymer.

High temperatures favor the diffusion rate of additives, as a consequence of a higher chain flexibility. During accelerated aging, Method 1 samples were exposed to a higher temperature than Method 2 samples, due to radiation contribution (UV-Vis region). In Method 1, the surface temperature of the samples reached a maximum value of 70°C, what might have contributed, in a more significant way, to the additive diffusion process to the surface of the samples, aged by this method. In Method 2, the maximum temperature was 63°C.

The CI was determined according to equation (1). In Table 2, it is shown the CIs as a function of aging time for both methods.

Table 2.

CI values for samples before and after accelerated aging by Methods 1 and 2.

Method
Method	200	1000	2000
1	0	0.3	0.4
2	0	0.1	0.2

It can be observed, from Table 2, that the carbonyl group is not present in samples non-aged and aged for 200 h, independently of the aging method. Nevertheless, in samples, aged for 1000 h and 2000 h, the carbonyl is present, the CI being greater for samples aged by Method 1, suggesting a higher level of oxidation of the samples aged by this method. It is also noted that the CI values increase significantly between 200 h and 1000 h, such increase being more significant in Method 1.

In the study of Gulmine and collaborators,²⁸ stabilized HDPE samples were exposed to accelerated aging, up to 1600 h, at the same temperature, humidity, and radiation used in Method 2. By FTIR, a CI value of 0.2, for 1600 h, was obtained. This value is pretty much close to the one obtained, in the present study, for sample B3 (2000 h of accelerated aging).

Passador and collaborators²⁹ and Carrasco and co-authors³⁰ studied the artificial aging of HDPE and suggested that chemical modifications such as the formation of carbonyl and vinylic groups would be relevant only for exposure time superior to 60 days (1440 h). In the present study, carbonyl groups were not identified in samples exposed for 200 h (8.33 days) of aging, but in samples exposed to 1000 h (41.67 days), these groups could be identified. Also, in the analyzed samples, vinylic group bands were not identified (vinylidenes—absorption at 888 cm⁻¹ and vinyl groups—absorption at 910 cm⁻¹ and at 990 cm⁻¹, trans-vinylenes—absorption at 964 cm⁻¹. This result suggests that there was no crosslinking during the accelerated aging process.

Therefore, the obtained FTIR results point out that the HDPE of the samples underwent degradation with time and according to the aging method used in the present study, resulting on the formation of hydroperoxide and carbonyl, being more intense for Method 1. It seems that the difference in humidity condition contributed to the higher incidence of hydroperoxides in samples aged by method 1.

Differential scanning calorimetry analysis

Differential scanning calorimetry curves of the samples, before and after accelerated aging by Methods 1 and 2, for the determination of OOT (exothermic event) are shown in Figure 2. DSC curves for the determination of crystalline melting temperature (T_m) and of crystalline melting enthalpy (ΔH_m) are shown in Figure 3. These results refer to DSC first run. OOT values, T_m, and ΔH_m are presented in Table 3.

Figure 2.

Differential scanning calorimetry curves (OOT) for samples before and after accelerated aging: (a) Method 1 and (b) Method 2.

Figure 3.

Differential scanning calorimetry curves (Tm) for samples before and after accelerated aging: (a) Method 1 and (b) Method 2.

Table 3.

OOT, Tm, and ΔHm values for samples before and after accelerated aging by Methods 1 and 2.

Method	Aging time (h)	Sample	OOT (°C)	T_m (°C)	ΔH_m (J.g⁻¹)	Crystallinity degree (%)
1	0	A0	276	132	156	53
	200	A1	241	130	158	54
	1000	A2	224	132	163	56
	2000	A3	218	135	153	52
2	0	B0	276	136	172	59
	200	B1	270	133	169	58
	1000	B2	256	137	171	58
	2000	B3	220	134	149	51

In Table 3, a comparison between OOT results for samples, before and after accelerated aging, by Methods 1 and 2, can be seen. We observed a reduction in the OOT with time, for both methods. Also, it is observed that the lowest values for this temperature are for samples aged for 2000 h, independently of the method. The OOT values decrease for samples aged for 200 h and 1000 h, in comparison with the non-aged sample, were more significant in Method 1 than in Method 2. For 200 h of aging, there was a 35°C decrease (13%), in Method 1, compared to a 6°C decrease (2%), in Method 2. For 1000 h of aging, there was a 52°C reduction (19%) in OOT value, in Method 1, while, in Method 2, this reduction was 20°C (7%). These results suggest that the concentration of antioxidants at the surface of the samples aged by Method 1 is, probably, lower than at the ones aged by Method 2, at least considering the aging times of 200 h and 1000 h. This would be a consequence of the more aggressive weathering conditions imposed by Method 1, when compared to Method 2. The migration and consequent loss of additives hypothesis, based on the FTIR results, corroborate the analysis based on the OOT results.

Gulmine and collaborators²⁸ reported a reduction, around 10%, in the OOT value (240°C), for a sample aged for 800 h, under similar conditions used in Method 2 of the present work. In our case, for sample B2 (1000 h of aging), a reduction of 7% in the OOT value of the non-aged sample (276°C) was observed.

After 2000 h of accelerated aging, the OOT values obtained by both methods are practically the same (218°C and 220°C, for Methods 1 and 2, respectively). It suggests that the concentration of antioxidants at the region where the samples were taken reached similar values, at 2000 h of aging, independently of the aging method and of the sample shape. According to Volponi,³¹ the reduction if the OOT is directly related to the consumption of additives and/or to the degradation of the polymer. The results for the OOT, obtained in the present work, point out to a consumption of additives for the samples aged up to 2000 h, converging to a same value, for both methods. Nevertheless, the reduction if the OOT for the samples aged up to 1000 h was much more significant (higher rate) when Method 1 was used, compared to Method 2. According to CI values, calculated by FTIR spectra (Figure 1), it can be inferred that sample A3 seems more degraded than sample B3. As the OOT values for both samples (A3 and B3) are practically the same, it implies that the greater degradation of sample A3 might well be related to the more intense consumption of additives, during the first 1000 h of aging.

The crystallinity degree, calculated from the ΔH_m values, obtained from DSC curves (Figure 3), as a function of aging time, is also presented in Table 3, where it can be observed that the values of the crystallinity degree of the samples aged by Method 2, for 0 h, 200 h, and 1000 h, are slightly superior to the ones obtained for the samples aged by Method 1. Such result is probably related to the specimen’s preparation. The samples used in Method 2 were subjected to processing (melting dumbbell-shaping insulators specimens, according to ASTM D638), while the ones used in Method 1 were taken directly from the polymeric insulator surface, without any further processing. There is no significant difference in the crystallinity degrees of the samples aged for Method 1, and it presented a crystallinity degree between 52% and 56%. This behavior is the same when compared with samples, aged until 1000 h, using Method 2, presented a crystallinity degree of the 58% and 59%. From 1000 h to 2000 h of aging, by Method 2, there can be seen a reduction in the crystallinity degree of the samples, decreasing from 59% to 51%. In Table 3, it is worthwhile to observe the convergence of the crystallinity degree of the samples, aged by both methods, at 2000 h of aging. This result can be interpreted as a decrease, with aging, of the thermal history influence on the crystallinity. Such convergence was also observed concerning the OOT. Carrasco and collaborators³⁰ observed a small and random variation of the crystallinity degree of HDPE as a function of aging time for a 120 days period (2880 h). The authors concluded that there is little influence of the UV radiation on the crystallinity degree of HDPE, a result similar to that found in the present study.

As it can be observed in Table 3, the crystalline melting temperature of the aged samples presented a small variation compared to the respective non-aged sample. Therefore, as observed for the crystallinity degree, the crystalline melting temperature was little influenced by the aging method and by the sample shape.

Rheometric analysis

The M_w and polydispersity index (PI) as a function of aging time, for samples aged by Methods 1 and 2, can be seen in Table 4.

Table 4.

M_w values and PI for samples before and after accelerated aging by Methods 1 and 2.

Method	Aging time (h)	Sample	Mw (g.mol⁻¹) × 10⁵	PI
1	0	A0	2.1	2.3
	200	A1	2.5	2.0
	1000	A2	2.6	2.2
	2000	A3	2.7	2.8
2	0	B0	2.1	2.3
	200	B1	2.4	2.0
	1000	B2	2.6	2.0
	2000	B3	2.1	2.0

From this Table, it can be observed that the order of magnitude of the M_w for all samples is the same, that is, 10⁵. The molar mass for the samples varied from 2.1 × 10⁵ g mol⁻¹ to 2.7 × 10⁵ g mol⁻¹, which is not significant. This result suggests that there was no considerable degradation of the samples. Although the FTIR results pointed to the occurrence of degradation, for 1000 h and for 2000 h, considering the formation of carbonyl and hydroperoxide groups, it seems, to the results of the rheometric tests, that the accelerated aging induced just a mild degradation of the samples, affecting mainly the side groups, with no significant effect on the molar mass. In Table 4, it can be observed a small variation of the PI up to 2000 h, especially for Method 2. For this method, the PI varied from 2.0 to 2.3, while for Method 1, it varied from 2.0 to 2.8. This result supports the inference of a mild degradation of the samples, as it indicates a variation of the molar mass, but not a significant one.

In addition, from the frequency sweep test, it was observed that, for samples A0 and A1 and all samples of Method 2 (B0, B1, B2, and B3), at lower angular frequencies, up to the crossover point, the viscous behavior predominated over the elastic one (G" > G'). For samples A2 and A3, the elastic behavior predominated over the viscous behavior (G' > G"), along the whole angular frequency interval evaluated. Samples aged by Method 1 presented a gradual increase in the elastic contribution, what was not observed for samples aged by Method 2. This result suggests that the two different methods used for HDPE samples aging result in different aging effects over the samples.

Hoekstra and collaborators³² observed in HDPE samples, in conditions similar to those used in Method 2, that the molar mass of the samples remained at the same order of magnitude, with the PI decreasing from 3.0 to 2.4. In the study of Maria and collaborators,³³ HDPE-aged samples did not present any significant change in molar mass and their PI remained the same.

Conclusions

The accelerated aging conditions, used in Methods 1 and 2, affected the aging of HDPE pin type insulators samples differently, the changes being, as expected, more significant in Method 1, considering that it is more aggressive than Method 2. Both methods led to sample oxidation, after accelerated aging for 1000 h. A reduction, up to 21%, if the OOT, after 2000 h of accelerated aging, was observed. The T_m and the crystallinity degree were little affected by aging, independently of the used method and of the sample shape. The M_w remained at the same order of magnitude, while the PI presented a small variation. The obtained results suggest the occurrence of additive migration to the surface of the samples, followed by an additive loss, as a function of accelerated aging time. Finally, it is important to emphasize, as can be observed from the obtained results, that, with aging, the polymer undergoes a significant change on its structure–property relationship.

Footnotes

Acknowledgments

The authors thank PETROBRAS, CAPES, and CNPq for their technical support.

Declaration of conflicting interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The author(s) disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: This work was supported by the CEMIG/TAESA grant numbers 4020000609 – D-447 and FAPEMIG grant numbers APQ-03259-11 – D-530.

ORCID iD

Cynthia DC Erbetta

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