Influence of Antioxidants on Heterocharge Generation in Polymeric Dielectrics

HIROKAZU SUZUKI,1 YASUO SEKII,2 KAZUO NOGUCHI,1 KOICHI SHIMURA,1 andTAKASHI MAENO3

1Chiba Institute of Technology, Japan2Sekii Consulting Engineers Office, Japan

3National Institute of Information and Communication Technology, Japan

SUMMARY

The generation of space charges in polymeric dielec-tric materials is known to be affected by such inclusions ascross-linking by-products, antioxidants, and moisture. Wereported previously that negative heterocharges in XLPEare generated in the presence of acetophenone and sulfur-containing antioxidants. In this paper we describe furtherexperiments using samples containing DCP and three dif-ferent antioxidants in order to examine the mechanism ofthis phenomenon. Negative heterocharges near the positiveelectrode were detected in samples containing both DCPand antioxidants when those samples were soaked in ace-tophenone. Negative heterocharges were not detected insamples containing either DCP or the antioxidant, evenwhen the sample was soaked in acetophenone. These resultsdemonstrate that a heterocharge is generated by the com-bined effect of acetophenone and the substance producedby interaction between DCP and the antioxidant. Based onthe results of experiments measuring the amount of hetero-charge in samples that had been heatpressed at differenttemperatures, the mechanism of negative heterocharge gen-eration in polymeric dielectric material is inferred. © 2010Wiley Periodicals, Inc. Electr Eng Jpn, 172(2): 1–9, 2010;Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/eej.20941

Key words: dielectrics; space-charge; antioxidant;DCP; acetophenone.

1. Introduction

XLPE cables using cross-linked polyethylene(XLPE) are widely used in AC electric power transmission.DC power transmission makes use of OF (oil filled) cables,mass-impregnated cables, because of space-charge prob-lems. When a space charge accumulates in an insulation

layer, a local high-electric field is formed, so that insulationperformance drops and insulation breakdown occurs.

In XLPE cable using cross-linked polyethylene as theinsulation layer, the accumulation of space charges isknown to occur readily. Research on the mechanisms thatform space charge and methods of preventing the accumu-lation of space charges have been undertaken at variousplaces. Our group has been involved in this research, inparticular by performing various experiments in order tounderstand the mechanism of formation of negative hetero-charges that influence the insulation performance of cables.Based on prior research, we have clarified the effects of thecross-linking agent DCP (dicumyl peroxide) and its by-products, as well as antioxidants, on space charge formation[1]. In particular, we have confirmed in our recent researchthat negative hetero-charges are formed when acetophe-none, a sulfur-containing antioxidant, or cross-linking by-products are present in XLPE [2]. We performed thefollowing experiments in order to further elucidate thisresult:

(1) an experiment on samples with antioxidantsadded;

(2) an experiment investigating the relationship be-tween antioxidants and DCP;

(3) an experiment in which the heat-press tempera-ture in creating the sample was varied.

Based on the results of the experiments, we consider themechanism responsible for the generation of the negativeheterocharge and present an analysis of the results.

2. Samples and Experimental Methods

2.1 Antioxidants used

We used cross-linking agent DCP and the four kindsof antioxidants listed in Table 1. In Table 1, F1 and F2 are

© 2010 Wiley Periodicals, Inc.

Electrical Engineering in Japan, Vol. 172, No. 2, 2010Translated from Denki Gakkai Ronbunshi, Vol. 128-A, No. 4, April 2008, pp. 216–222

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phenolic antioxidants, and S1 and S2 sulfur-type antioxi-dants. F1, S1, and S2 all include sulfur atoms in theirmolecular structure.

2.2 Preparation and pretreatment of thesamples

The sample preparation procedure is as follows.

(1) Weigh the LDPE pellet, antioxidants, and DCP(antioxidants 0.3%, DCP 2%).

(2) Use a mixing roll for mixing after adding theantioxidants and DCP to the LDPE pellet. The mixing timeis approximately 20 minutes, so that the added materials arespread uniformly.

(3) With a heating press, apply pressure to the mixedcompound at a temperature of 115 °C and a pressure of 200kgf/cm2 for 20 minutes (as a result, the sample thicknessreaches about 1 mm).

(4) Because the primary product of step (3) alone maybe too thick, press for 20 minutes at a maximum of 400kgf/cm2 at a temperature of 115 °C, then perform secondarymolding. As a result of this secondary molding, the samplethickness can be brought to about 0.3 mm. The LDPEsample is created by steps (1) to (4).

(5) The pressed sheet formed by secondary moldingof the XPLE sample is further pressed for 20 minutes whileapplying a pressure of 400 kgf/cm2 at 180 °C in order toachieve cross-linking (tertiary molding). As a result, thethickness of the XLPE sample reaches 0.3 mm. After the

pretreatment listed in Table 2 the space charge distributionin samples is measured.

2.3 Method to measure space-chargedistribution

We used the pulsed electrostatic method (PEAmethod) to measure the space charge distribution. Figure 1shows the measurement circuit. In the measurements, a DCelectric field of 20 kV/mm was applied, and the duration ofvoltage application was 10 minutes. The measurementswere performed five times.

3. Experiment on Effect of Sulfur-ContainingAntioxidants

3.1 Materials

The samples listed in Table 3 were used in thisexperiment.

3.2 Experimental results and discussion

Figures 2 to 5 show the results of the experiment. Ascan be clearly seen in Figs. 2 to 5, a negative heterochargeoccurs in all the untreated XLPE samples (labeled NT

Table 2. Pretreatment of samples

Table 1. Added cross-linking agent and antioxidant

Fig. 1. Space-charge measuring circuit.

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sample), and also in the “D+AP” treated samples except forthe F2-containing sample. Among the LDPE samples, it isclear that a negative heterocharge occurs in all “D+AP”samples. Table 4 shows whether an antioxidant (A.O) oracetophenone (AP) is included.

As can be seen in Table 4, a negative heterochargeappears in all of the NT (untreated) samples in XLPE,apparently because acetophenone and moisture are presentin the samples. Furthermore, even in the LDPE sampleswithout acetophenone, if “D+AP” acetophenone impregna-tion treatment was performed after drying, the presence ofa negative heterocharge was confirmed. Furthermore,among the XLPE samples subjected to “D+AP” treatment,

Fig. 2. Charge profiles of 0.3% F1 containing XLPEand LDPE.

Fig. 3. Charge profiles of 0.3% F2 containing XLPEand LDPE.

Table 3. Details of samples

Fig. 4. Charge profiles of 0.3% S1 containing XLPEand LDPE.

Fig. 5. Charge profiles of 0.3% S2 containing XLPEand LDPE.

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it is clear that a negative heterocharge appeared in threesamples, excluding only the sample containing F2.

4. Relationship Between Antioxidants and DCP

As was explained in Section 3, when dried LDPEsamples with the antioxidants F1, F2, S1, and S2 added werethen impregnated with acetophenone, the presence of anegative heterocharge was confirmed. In order to analyzethe mechanism of this heterocharge formation, we preparedsamples with only the antioxidants F1, F2, S1, S2, or DCPadded, then performed an experiment investigating thespace charge distribution.

4.1 Samples

Table 5 lists the samples used in the experiment. Thesamples were all LDPE: sample “a” contained only DCP,and samples “b” to “e” contained only an antioxidant.

Before measuring the space charge distribution, we per-formed “D+AP” pretreatment of all samples.

4.2 Results of experiments

Figure 6 shows the results of the experiments. As canbe seen in the figure, we confirmed that when neither DCPnor an antioxidant is added, no heterocharge appears evenin the samples containing acetophenone.

4.3 Remixing experiment

In order to further confirm the results in Section 4.2,we remixed the samples shown in Table 5 by adding DCPor an antioxidant if one had not been originally added, theninvestigated the space charge distribution of the samplesthat included both DCP and antioxidants. Table 6 shows thesamples used. “D+AP” treatment was performed on all ofthe samples. Figures 7 and 8 show the measurement results.Figure 7 represents the results for the samples (AF1, AF2,AS1, AS2) obtained by remixing with addition of an antioxi-dant to sample “a,” and Fig. 8 represents the results for thesamples (B, C, D, E) obtained by remixing in which DCPwas added to samples “b,” “c,” “d,” and “e.” As is evidentin Figs. 7 and 8, a negative heterocharge was observed inall of the samples. Table 7 summarizes the experimental

Table 4. Summary of charge profile measurements

Fig. 6. Charge profiles of LDPE samples containingeither DCP or A.O.

Table 5. Details of samples

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results shown in Figs. 6 to 8. As shown in Table 7, weconfirmed that when the samples with antioxidants F1, F2,S1, S2, and DCP added were impregnated with acetophe-none, a negative heterocharge appeared.

5. Experiment with Variation of Sample FormationTemperature

As explained in Section 4, it is clear that when noDCP is added, even if a sample with an antioxidant addedis impregnated with acetophenone, no negative hetero-charge appears. Figure 9 shows the thermal decompositionreaction of DCP. It can be seen that acetophenone is formedby the thermal decomposition reaction of DCP.

We hypothesize that when an antioxidant is added tothe LDPE pellet, a chemical reaction, that is different fromthe DCP thermal decomposition reaction shown in Fig. 9,between an antioxidant and DCP occurs, and the productmay form a negative heterocharge in the presence of aceto-

Fig. 8. Charge profiles of 0.4 gr DCP-added samples.

Table 6. Samples used for the experiment Table 7. Summary of experimental results

Fig. 7. Charge profiles of 0.6 gr A.O.-added samples.

Fig. 9. Thermal decomposition reaction of DCP.

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phenone. In order to verify this hypothesis, we measuredthe space charge distribution of samples prepared by vary-ing the press temperature and leaving some samples un-treated (NT) while applying “D+AP” treatment to theothers. We then attempted to find the maximum volumeconcentration of the negative heterocharge in the sample.

5.1 Samples and experimental results

The samples were created by varying the press tem-perature over a range of 115 °C (120 °C) to 200 °C in amixture of 2.0% DCP and 0.3% F2, S1, or S2 added to apolyethylene pallet. Among the added antioxidants, F2 is aphenol antioxidant that does not include sulfur atoms in itsmolecular structure, and S1 and S2 are sulfur-containingantioxidants. Figures 10 to 12 show the space charge distri-butions of the sample with S1 added, prepared at presstemperatures of 115, 155, and 200 °C. Because the resultscannot all be displayed due to space limitations, in thispaper the distribution for sample S1 prepared at the threetemperatures described above is shown as an example ofthe space charge distribution obtained. Based on the experi-mental results, in the sample prepared at a press temperaturebelow 140 °C, no heterocharge was found in the untreatedsample (NT sample). On the other hand, in the sample with“D+AP” treatment, a heterocharge was observed when thepress temperature was low. But when the press temperatureexceeded some level, the heterocharge density in the sampletended to decrease.

5.2 Discussion of experimental results

Based on the measurement results for the spacecharge distribution, Figs. 13 to 15 show a plot of therelationship between the press temperature and the maxi-mum volume charge density of the heterocharge in thesample. These figures confirm that the maximum volumedensity of the detected heterocharge varied with the presstemperature of the sample, even when an antioxidant wasadded. Furthermore, it is clear that the nature of the changevaried significantly between the untreated sample (NT sam-ple) and the treated sample. A heterocharge began to appear

Fig. 10. Charge profile of sample heat-pressed at 115°C.

Fig. 11. Charge profile of sample heat-pressed at 155 °C.

Fig. 12. Charge profile of sample heat-pressed at 200 °C.

Fig. 13. Relations of press temperature and negativeheterocharge density (F2 added samples).

Fig. 14. Relations of press temperature and negativeheterocharge density (S1 added samples).

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in the untreated samples (NT samples) at a press tempera-ture of about 155 °C, and the maximum volume density ofthe heterocharge clearly rose as the press temperature wasincreased. The same trend was seen even when an antioxi-dant was added.

On the other hand, the way in which the maximumvolume density of the charge varied as a result of the presstemperature in the sample with “D+AP” treatment changedsubstantially due to the added antioxidant. In the samplewith the phenol antioxidant F2 which contains no sulfuratoms, the highest volume density of the heterochargereached a maximum at a press temperature of 120 °C. Itthen dropped as the press temperature rose, and no hetero-charge was observed in samples pressed at above 160 °C.In contrast, the volume density of the charge rose along withthe press temperature until about 155 °C for the sampleswith sulfur-containing antioxidants S1 and S2. It is clear thatwhen a temperature of approximately 155 °C is exceeded,the volume density of the charge starts to decrease.

Figure 16 shows a plot of the relationship betweenthe cross-linking temperature and the amount of cross-link-ing by-products generated in the cross-linking reaction dueto DCP (DCP 2%; S1 0.2%) [3]. This graph indicates thatacetophenone begins to be formed around 150 °C, and thatthe amount formed increases as the cross-linking tempera-ture is raised. In the plots of the maximum volume densityof the heterocharge shown in Figs. 13 to 15, the negativeheterocharge began to appear at a press temperature ofabout 155 °C in the NT sample, and the maximum volumedensity also rose as the temperature increased. The resultsof the NT sample show a strong relationship betweenheterocharge formation and the formation of acetophenonewhich begins at about 155 °C. Because the amount ofacetophenone generated increases as the temperature rises,the volume density of the charge also increases.

On the other hand, it seems likely that the substancesgenerated by reaction between the antioxidant and DCP inthe “D+AP” samples generates a heterocharge in the aceto-

phenone-impregnated samples as a result of “D+AP” treat-ment. The chemical reaction between the antioxidant andDCP becomes stronger as the temperature rises as far as 155°C. However, when 155 °C is exceeded, the thermal decom-position reaction of DCP becomes dominant. As a result,the amount of DCP contributing to the reaction with theantioxidant diminishes, and formation of heterocharge inthe sample with F2 added disappears, and the heterochargein the samples with S1 or S2 added decreases.

When “D+AP” treatment is applied to a sample pre-pared at a press temperature in the range below 160 °C, theobserved heterocharge generation behavior varies substan-tially in the samples with F2 added versus the samples withS1 or S2 added. This result suggests that the mechanism offormation of the substances involved in heterocharge for-mation varies between the sample with F2 added and thesamples with S1 or S2 added. This difference appears to berelated to the difference in the molecular structure of theantioxidants, and in particular to whether sulfur atoms arepresent in the molecules. In experiments conducted pre-viously, one of us confirmed that sulfur-containing antioxi-dants decompose at high temperatures, with the formationof sulfuric acid, sulfonic acids, and other acids [3]. Theresults of previous experiments suggest that DCP reactswith antioxidants S1 and S2, and that a substance capable ofdissociating into ions is formed. It appears that this gener-ated substance forms heterocharges in the presence of ace-tophenone.

6. Conclusions

In order to evaluate the effects of antioxidants onheterocharge formation, we used four antioxidants, andperformed experiments to investigate the effects of antioxi-

Fig. 15. Relations of press temperature and negativeheterocharge density (S2 added samples).

Fig. 16. Relationship between cross-linkingtemperature and amounts of cross-linking by-products.

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dants on the formation of the heterocharge generated nearthe anode in low-density polyethylene (LDPE) and cross-linked polyethylene (XLPE). The main conclusions of thispaper are as follows.

(1) We confirmed that when acetophenone is presentin a low-density polyethylene (LDPE) or cross-linked poly-ethylene (XLPE) sample heated and molded with antioxi-dant added, a negative heterocharge is formed, and anegative heterocharge is observed in XLPE when sulfur-containing antioxidants are added.

(2) The heterocharge noted in item (1) above appearsto be a result of a substance formed by the reaction of anantioxidant and the cross-linking agent DCP. Furthermore,the chemical reaction between the DCP and the antioxidant

appears to vary depending on whether or not sulfur atomsare included in the molecules of the antioxidant.

REFERENCES

1. Obayashi T, Uchimura T, Fukuyama T, Sekii Y,Maeno T. Effect of moisture on heterocharge forma-tion in XLPE. Trans IEE Japan 2002;122-A:171–177. (in Japanese)

2. Sekii Y, Taya A, Maeno T. Experiments on chargecreation in cross-linked polyethylene and ethylene-propylene copolymer. IEEJ Trans FM2007;127:212–218. (in Japanese)

3. Sekii Y, Idei Y, Asagawa K, Maeno T. Influence ofantioxidants on polyethylene chemical crosslinkingreaction. Annual Report of 2001 CEIDP, p 83–87.

AUTHORS (from left to right)

Hirokazu Suzuki (student member) graduated from the Department of Electrical, Electronic and Computer Engineering,Chiba Institute of Technology, in 2007 and entered the electrical, electronic and computer engineering course of the GraduateSchool of Engineering there. He is primarily engaged in research on the generation of space charges in electrical insulationmaterials. He received the 2007 “Electrical Academic Scholarship Award” of the Institute of Electrical Engineers of Japan TokyoDivision.

Yasuo Sekii (member) completed the M.E. program at the University of Tokyo in 1965 and joined Hitachi Cable, Ltd. Heserved as the General Manager of the First Department of the Power Systems Laboratory there, then as chief of research. Hewas engaged in research and development related to DC and AC high-voltage XLPE cable systems and in research on thedegradation of insulating materials. From 1994 to 2007 he was a professor at Chiba Institute of Technology. From 2007 to 2009he served as the chairman of the Japan Electric Cable Technology Center (JECTEC). He is a licensed profession engineer(electrical and electronic division). He holds a D.Eng. degree, and is a member of the Institute of Electrostatics of Japan and anIEEE Life Fellow.

Kazuo Noguchi (nonmember) completed the doctoral program (academic) at Shinshu University in 1995. He is now anassociate professor in the Department of Electrical, Electronic and Computer Engineering at Chiba Institute of Technology. Hiscurrent research interests include the measurement of atmospheric aerosols using laser radar, electrical conduction phenomenaof insulating materials, and the diagnosis of electrical apparatus by electromagnetic wave detection. He is a member of the JapanSociety of Applied Physics, IEICE, and the Laser Society of Japan.

Koichi Shimura (nonmember) began studies in the Department of Electrical, Electronic and Information Engineering,Chiba Institute of Technology, in 2004.

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AUTHORS (continued)

Takashi Maeno (member) completed the doctoral program in electrical engineering at Musashi Institute of Technologyin 1989. He is now affiliated with the Electromagnetic Wave Measurement Research Center ESNG at the National Institute ofInformation and Communications Technology. Since 2007 he has been a professor in the Graduate School of Musashi Instituteof Technology. He is primarily engaged in research on measurement of space charge distributions in dielectrics, measurementof electric field distributions, and laser applications. He received a 1990 IEEJ Outstanding Paper Award, a 1996 IEEJ ProgressPrize, and a 1997 Progress Prize of IEJ (Institute of Electrostatics Japan).

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