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Mechanism of antioxidant interaction on polymer oxidation by thermal and radiation ageing
Radiation Physics and Chemistry 81 (2012) 1747–1751
Contents lists available at SciVerse ScienceDirect
Radiation Physics and Chemistry journal homepage: www.elsevier.com/locate/radphyschem
Mechanism of antioxidant interaction on polymer oxidation by thermal and radiation ageing$ Tadao Seguchi b,n, Kiyotoshi Tamura b, Akihiko Shimada a, Masaki Sugimoto a, Hisaaki Kudoh c a
Japan Atomic Energy Agency (JAEA), Takasaki Applied Quantum Chemistry Institute, Takasaki 370-1292, Japan JAEA, Atomic Science Institute, Tokai 319-1195, Japan c The University of Tokyo, Japan b
H I G H L I G H T S c c c c c
Interaction of antioxidant on polymer oxidation is discussed for thermal and radiation ageings. Antioxidant is very effective for thermal oxidation, but not for radiation induced oxidation. Interaction of antioxidant is not the termination reaction of radicals on polymers. Antioxidant is supposed to reduce the provability of polymer radical formation by thermal activation. Mechanism of polymer oxidation may not be chain reaction via peroxy radical and hydro-peroxide.
a r t i c l e i n f o
abstract
Article history: Received 9 May 2012 Accepted 18 June 2012 Available online 3 July 2012
The mechanism of polymer oxidation by radiation and thermal ageing was investigated for the life evaluation of cables installed in radiation environments. The antioxidant as a stabilizer was very effective for thermal oxidation with a small content in polymers, but was not effective for radiation oxidation. The ionizing radiation induced the oxidation to result in chain scission even at low temperature, because the free radicals were produced and the antioxidant could not stop the oxidation of radicals with the chain scission. A new mechanism of antioxidant effect for polymer oxidation was proposed. The effect of antioxidant was not the termination of free radicals in polymer chains such as peroxy radicals, but was the depression of initial radical formation in polymer chains by thermal activation. The antioxidant molecule was assumed to delocalize the activated energy in polymer chains by the Boltzmann statics (distribution) to result in decrease in the probability of radical formation at a given temperature. The interaction distance (delocalization volume) by one antioxidant molecule was estimated to be 5–10 nm by the radius of sphere in polymer matrix, though the value would depend on the chemical structure of antioxidant. & 2012 Elsevier Ltd. All rights reserved.
Keywords: Mechanism Degradation Polymer Oxidation Antioxidant Radiation
1. Introduction Antioxidants are essential stabilizers for most of the polymer materials. The polymer degradation is significantly reduced by mixing a small content of antioxidant as is well known. For the lifetime extension of polymer materials, many types of antioxidants have been developed for their application in various environments. However, the information of specific antioxidants (name and the content) in the practical polymer materials has been the knowhow of the processing company, because, the $ The research was under the contract between NISA (Nuclear and Industrial Safety Agency, Japan) and JAEA. n Corresponding author. E-mail address: [email protected] (T. Seguchi).
0969-806X/$ - see front matter & 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.radphyschem.2012.06.011
information of formulation (compositions of stabilizers and fillers) are the key points to determine the properties and life time of materials. The antioxidant is a miner content in polymer formulation; it is about 0.5–1.0 phr (parts per hundred resin) for crystalline polymers, and 1.0–2.0 phr for rubbers in the practical polymer materials. For most of the polymers as raw materials, a small content of stabilizer (around 0.05 phr of antioxidant) was mixed immediately after the polymerization, because without stabilizer the oxidation of polymer proceeds in air even at ambient temperature. The antioxidants are organic compounds and there are many species, which are mainly 4 groups as phenol, amine, sulfur, and phosphine (Haruna, 2010). From vast studies the oxidation schema have been established to be the chain reaction via peroxy radical and hydro-peroxide, where the role of antioxidant for polymers was the termination of
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T. Seguchi et al. / Radiation Physics and Chemistry 81 (2012) 1747–1751
free radicals or decomposition of hydro-peroxides in polymer chains under oxidation process (Conley, 1970; Osawa, 1992). In above schema the decrease of antioxidant content in polymer matrix during oxidation was thought to be due to the consumption in the reaction with radicals. In the studies of cable degradation for nuclear power plants by thermal and radiation ageing, the effect of antioxidant was found to be significantly different between thermal and radiation oxidations (Seguchi et al., 2011). A small amount of antioxidant was very effective for thermal oxidation, but not for radiation oxidation. The decrease of antioxidant content was supposed to be due to the evaporation in thermal oxidation, and to the decomposition in radiation oxidation. For both oxidations the primal oxidation products were the carboxylic acid formed by polymer main chain scission (Sugimoto et al., 2012). These findings indicated that the oxidation mechanisms were different from the chain reaction via peroxy radical and hydro-peroxide, and also the role of antioxidant was completely different from those in the traditional reaction schemas. In this paper, the tensile elongation behavior of crosslinked polyethylene (XLPE) containing different antioxidants was compared by thermal and radiation oxidations. The role of antioxidant in polymer oxidation was discussed, and a new schema of antioxidant interaction in polymer matrix was proposed.
2. Materials and methods The polymer materials were the crosslinked polyethylene (XLPE) containing a specific antioxidant with different contents. XLPE-A was supplied from a cable company A, and the antioxidant was 2-mercapto-benzo-imidazole (Nocrac-MB). The antioxidant content was 0.0 (no mixing), 0.1, and 1.0 phr (part per hundred resin). XLPE-B was supplied from a cable company B, and the antioxidant was 4,4-Thio-bis(3-methyl-6-t-butyl phenol) (Nocrac-300). The content was the same with XLPE-A. Both XLPEA and -B were 1 mm thick sheet specimens by chemical crosslinking after mixing the respective antioxidant. The chemical structure of antioxidants for XLPE-A and -B is shown in Fig. 1. The chemical crosslinking and the stabilizers for both sheets were the same with practical cable insulation except the antioxidant. As a reference of practical formulation of XLPE, a raw polyethylene was examined. The raw polyethylene was of high molecular weight and contained a small amount of stabilizer, but the content and type were not revealed by the polymer company as usual. The specimens were 1 mm thick sheet by hot pressing.
Fig. 1. Molecular structure of antioxidant mixed in XLPE-A (Nocrac-MB) and XLPE-B (Nocrac-300).
The oxidation of the sheet specimens was carried out by radiation ageing and by thermal ageing in air. For radiation ageing, the specimens were irradiated by Co-60 gamma rays in air at 100 1C with a dose rate of 1.0 kGy/h using a thermostat oven. Here, the irradiation was carried out at 100 1C with a dose rate of 1 kGy/h in order to spread the radiation induced oxidation throughout the XLPE sheet of 1 mm thickness. At 100 1C the rate of thermal oxidation is very small, so most of it is induced by radiation oxidation. For the thermal ageing, the specimens were stored at 100, 135, and 155 1C in air using a gear type oven. For the raw PE sheets, the thermal ageing was carried out at 95, 110, and 125 1C in air using same oven. The changes of material properties were measured by tensile test at room temperature. The sheet specimens were cut to dumbbell shape test pieces (Japan Industrial Standard 3-go; 100 mm length, 5 mm width at central part). The data were the average of 5 pieces except the abnormal data, and the reproducibility of 5 pieces was 710% for the elongation at break determined by photo-trace of the marker on test piece.
3. Results The thermal degradation, the decay of elongation at break, of raw polyethylene sheet is shown in Fig. 2. The change of the tensile strength was similar to that of elongation. A small amount of antioxidant was mixed in this polymer, so the oxidation during sheet processing above melting temperature was reduced. The oxidation proceeded by thermal ageing at 90 1C, and the elongation decreased rapidly over 100 h ageing time, and the time decreased with the increase of temperature from 110 to 125 1C. The rapid decay of elongation in Fig. 2 reflected the decrease of antioxidant in polymer matrix. Fig. 3 shows the effect of antioxidant content for XLPE-A (Nocrac-MB) degradation by thermal and radiation ageings. The initial values of XLPE-A (10 h or 10 kGy in Fig. 3) were a little different with the antioxidant content, which depended on the crosslinking density. The decays of elongation by thermal ageing at 135 1C were significantly different with the antioxidant (Nocrac-MB) content. With 0.1 phr content, the decay time extended to 10 times more than that of no Nocrac-MB (0.0 phr), but XLPE-A (0.0 phr) contained a small amount of antioxidant in the raw polymer and also the other stabilizers. By increase of antioxidant to 1.0 phr, the decay of elongation shifted to the right
Fig. 2. Elongation at break for raw polyethylene sheet by thermal ageing in air at 95, 110, and 125 1C.
T. Seguchi et al. / Radiation Physics and Chemistry 81 (2012) 1747–1751
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Fig. 3. Elongation at break of XLPE-A (Nocrac-MB: 0.0, 0.1, 1.0 phr) by thermal ageing at 35 1C and by radiation ageing.
N300 N3001.0phr 1.0 phr
N3001.0phr 1.0 phr N300
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N3000.1phr 0.1 phr N300
N3000.0phr 0.0 phr N300
N3000.0phr 0.0 phr N300
600
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Elongaon at break /%
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Fig. 4. Elongation at break of XLPE-B (Nocrac-300: 0.0, 0.1, 1.0 phr) by thermal ageing at 135 1C and by radiation ageing.
(longer ageing time), and was estimated by the dotted line to over 1000 h. In the case of radiation ageing, the decay of elongation was scarcely affected by the content of 0.1 phr. However, the decay of XLPE-A with 1.0 phr shifted to high dose by 4–5 times. The shift can be explained by the anti-rad effect of aromatic compound NocracMB, and not by the antioxidant effect, because the anti-rad effect increases proportionally with the aromatic content in polymers. The decay of elongation for XLPE-B (Nocrac-300) is shown in Fig. 4, whose data were the same ones in our previous paper (Seguchi, 2011). The dotted lines for 0.1 and 1.0 phr at the longer ageing time are the estimated decay curves by considering the decrease of antioxidant. The initial value of elongation depended on the crosslinking density like that of XLPE-A. The fact that the decay of elongation for Nocrac-300 is longer than that for NocracMB indicates that the evaporation of Nocrac-300 is lesser than that of Nocrac MB at 135 1C. The decays of XLPE-B with 0.1 phr and 1.0 phr by thermal ageing were extended to longer time than those of XLPE-A with Nocrac-MB. For the radiation ageing, the
effect of 0.1 phr Nocrac-300 was small, and the effect increased with 1.0 phr as well as Nocrac-MB. In Figs. 3 and 4, the elongation tends to increase with ageing time until the rapid decay for both thermal and radiation ageings, which reflects the break of network structure of XLPE by the chain scission even at the stage of shorter ageing time for both ageings. Figs. 5 and 6 show the degradation of flame retardant ethylene-propylene rubber (EPR) insulator in FR-PH-2.0-C cable, and silicone rubber (SiR) insulator in KGB-2.0-B cable by thermal and radiation ageings for very long period at relatively low temperature and low dose rate irradiation. The data were obtained by the research group of Japan Nuclear Energy Safety Organization (JNES) through the national project for assessment of cable ageing (Yamamoto and Minagawa,2009). Both insulators were materials for the cables of a nuclear power plant. In the thermal ageing for EPR, the decay curves of elongation were similar for the different temperatures, and the decay times shifted with temperature. For radiation ageing, the decay curves are also similar for the different dose rates; here, the major
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Fig. 5. Elongation at break of frame retardant EPR (FR-EPR) insulator in FR-PH-2.0 cable by thermal ageing at 100, 110, and 120 1C, and by radiation ageing at 80 1C with dose rates 18 and 100 Gy/h in air (plots of JNES data; Yamamoto and Minagawa, 2009).
SiR-B
135 0C
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Elongation at break/%
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18 Gy/h, 100 0C 100Gy/h, 100 0C
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Ageing time/h Fig. 6. Elongation at break of silicone rubber (SiR) insulator in KGB-2.0-B cable by thermal ageing at 100, 110, and 120 1C, and by radiation ageing at 80 1C with dose rates 18 and 100 Gy/h in air (plots of JNES data; Yamamoto and Minagawa, 2009).
degradation should be induced by radiation oxidation, because the thermal oxidation rate at 80 1C would be very small. Fig. 5 shows that the decay behavior was significantly different between thermal and radiation ageings for the practical polymer material. For XLPE insulation materials in various cables, a similar behavior with EPR in Fig. 5 was observed by the analysis of JNES data. The difference between thermal and radiation oxidation should be closely related to the content of antioxidant as well as those in Figs. 3 and 4. For SiR degradation in Fig. 5, the degradation behavior was the same between thermal and radiation ageings. The oxidation mechanism of SiR was different from that of the olefin polymers (EPR, XLPE), and the antioxidant was not necessary for SiR insulation material. Therefore, for polymers without concerning antioxidant the oxidation mechanism would be the same or similar between thermal and radiation ageings.
4. Discussion As seen in Figs. 2 and 3, a small amount of antioxidant was very effective for the XLPE degradation by thermal ageing. Even
for raw PE, a small amount of stabilizer was effective until about 100 h at a lower temperature of 95 1C. Generally the mechanical degradation as elongation at break depends on the degree of oxidation, but not proportional to the oxidation. The extent of oxidation would be proportional to the ageing time as far as the antioxidant remains sufficient in the polymer matrix. Otherwise, the oxidation of polymer materials should proceed by thermal ageing even if antioxidant remains enough content. Any antioxidant (organic compound) is released from the polymer matrix by evaporation during thermal ageing and the release rate depends on the ageing temperature. When the antioxidant content was decreased to a certain value, the oxidation rate increased rapidly with decreasing content below a limited content. The ageing time at the sharp decay of elongation in Figs. 2, 3, 4, and 5 indicated that the antioxidant content was decreased to less than the limited content. The time of the limited content depends on the initial content and molecular characteristics of the antioxidant such as molecular weight and compatibility with polymer matrix. For the practical materials containing various stabilizers and fillers, the rapid degradation due to the decrease of antioxidant content might be lessened by comparing with a simple formulation such as XLPE-A and XLPE-B. For radiation ageing at relatively low temperature, that is, the thermal oxidation is small, and the radiation induced oxidation should be proportional to ageing time (or dose). Antioxidants were essentially not effective for radiation induced oxidation. However, for XLPE with high content of antioxidant, the degradation time shifted to higher doses, which could be explained by the effect of radiation protection by aromatics in antioxidant. The evidence was observed in EPR degradation with various contents of antioxidant though the explanation was different (Baccaro et al., 1993). It was confirmed that the radiation induced oxidation is proportional to dose by the yield of oxidative products in polymers materials (Seguchi et al., 2011; Baccaro et al., 1993). So, when the oxidation is proportional to ageing time, the elongation change should show the same curves for the radiation ageing in Figs. 2–5. For example in SiR degradation, the thermal oxidation would be proportional to ageing time, so the decay curves are the same with radiation oxidation. The fact that the antioxidant has no effects on radiation induced oxidation means that the free radicals formed in polymer chains do not react with the antioxidant. Because the primal active sites were the free radicals at ambient or low temperature in the studies of radiation effects of polymers, and in the presence of oxygen, the radicals reacted with oxygen to result in chain scission and oxidative products even at room temperature (Arakawa et al., 1982). In the thermal oxidation, a small amount of antioxidant is very effective to reduce the oxidation and increase the efficiency with an excess of antioxidant, but sufficient oxidation proceeds to a certain extent with any type of antioxidant with sufficient content. Below the limited content of antioxidant, the rate of oxidation is accelerated with a decrease in the content. Therefore, the idea that the role of antioxidant is the termination of free radicals is not acceptable even in the thermal oxidation. A new concept of antioxidant interaction with polymer chains is the delocalization of activated energy in polymer matrix in thermal ageing. A model of the concept is illustrated in Fig. 7. One antioxidant molecule interacts with polymer molecules within a sphere of radius R wherein radical formation by thermal activation is reduced. The population of activated sites in polymer matrix is determined by the Boltzmann statistics, and a part of the activated sites would convert to radicals. The role of antioxidant is supposed to decrease the conversion from activated sites to radicals by delocalization of activated energy.
T. Seguchi et al. / Radiation Physics and Chemistry 81 (2012) 1747–1751
Antioxidant depress radical production in effective volume of polymer matrix by energy delocalization for thermal activation Polymer matrix
X : Antioxidant molecule
X R: 7nm
: radical
Effective volume of antioxidant. Radius of sphere is about 7nm
Fig. 7. Model of antioxidant interaction with polymer in thermal oxidation.
The interaction distance, radius R of the sphere, could be estimated from the limited antioxidant content, that is, the limited content covers the interaction over the polymer matrix. In the case of XLPE-B, 0.1 phr antioxidant was sufficient, and the limited content (at sharp decay of elongation) was estimated to be 0.02 phr. As the XLPE has 50% crystals and antioxidant can be mixed in amorphous part, the actual content in amorphous part is 0.04 phr for the limited content. The concentration of Nocrac-300 at 0.04 phr is 1.0 10 6 mol/cm3, and one molecule per 1.6 10 18 cm3 of amorphous polymer matrix. If the volume of interaction area is a sphere, the radius R is 7 10 9 m (7 nm). Though the radius may be different among the types of antioxidant, it might be supposed to be around 5–10 nm for various antioxidant. The content of stabilizer in raw polymers is around 0.05 phr of the rather low molecular weight antioxidant, which reduces the oxidation at ambient temperature as is well known. The radicals should be induced in the interaction volume with a certain probability by thermal ageing, and the radicals should react with oxygen even if enough amount of antioxidant is present. Therefore, any antioxidant could not completely stop the thermal oxidation. When the antioxidant content is lower than the limited value, the oxidation is accelerated in the outside of interaction volume. For radiation ageing, the radicals are induced in polymer chains by ionizing radiation of high energy (10–20 eV) at any temperature, and the radical yield is proportional to dose. The radicals react with oxygen without the effects of antioxidant. Therefore, the antioxidant does not affect the radiation oxidation. When the antioxidant has the aromatic compound in molecular structure, the radical yield might be reduced due to the radiation protection effect by aromatics.
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5. Conclusion On the cable degradation by accelerated thermal and radiation ageings, the effects of antioxidant to XLPE for the insulation material were investigated by tensile test. The antioxidant reduced extensively the thermal oxidation with a small content of 0.1 phr. But for radiation ageing, the effects of antioxidant were essentially nothing. The decay curves of elongation were different between thermal and radiation oxidations, which reflect the effect of antioxidant. Namely, the radiation oxidation is proportional to ageing time (or dose), but the thermal oxidation is accelerated when the antioxidant content decreased to less than a limited content. The decrease of antioxidant was induced by evaporation during thermal ageing, especially at higher temperature. Although the decrease occurred by radiation ageing, the oxidation was not affected by the decrease of antioxidant, because of no effect of antioxidant on radiation oxidation. Of course, apparently the oxidation should be accelerated during radiation ageing when the contribution of thermal oxidation increased during irradiation by the radiation decomposition of antioxidant. Based on the experimental facts and our previous data analysis, a new schema of oxidation of polymers, especially, the interaction mechanism of antioxidant was proposed. The essence of antioxidant effect was the reduction of radical yield as the oxidation initiator in polymer chains for thermal oxidation, and not to terminate the oxidation by the reaction with radicals. It was supposed that one molecule of antioxidant interacts with polymer chains in a sphere volume within 5–10 nm radius. The effect of a small amount of antioxidant for thermal oxidation could be explained by this model.
References Arakawa, K, Seguchi, T, Watanabe, Y, Hayakawa, N, 1982. Radiation-induced oxidation of polyethylene, ethylene–butene copolymer, and ethylene– propylene copolymer. J. Polym. Sci., Polym. Chem. Ed. 20, 2681–2692. Baccaro, S, Buontempo, U, D’atanasio, P, 1993. Radiation induced degradation of EPR by IR oxidation profiling. Radiat. Phys. Chem 42, 211–214. Conley, R.T, 1970. Thermal Stability of Polymers. Marcel Dekker. Haruna, T, 2010. Handbook of Additives for Polymer. CMC Publishing Co., Ltd. (Japanese). Osawa, Z, 1992. Degradation and Stabilization of Polymers. Musashino Kurieito, Co., Ltd. (Japanese). Seguchi, T, Tamura, K, Ohshima, T, Shimada, A, Kudoh, H, 2011. Degradation mechanisms of cable insulation materials during radiation–thermal ageing in radiation environment. Radiat. Phys. Chem 80, 268–273. Sugimoto, M., Shimada, A., Yoshikawa, M., Kudoh, H., Tamura K., Seguchi, T. Products analysis of polyethylene degradation by radiation and thermal ageing, in preparation. Yamamoto, T., Minagawa, T. 2009. The Final Report of Assessment of Cable Ageing for Nuclear Power Plants. JNES SS Report, JNES-SS-0903, available at: /http:// www.jnes.go.jp/english/katsudou/topics/activity_reports.htmlS.
Contents lists available at SciVerse ScienceDirect
Radiation Physics and Chemistry journal homepage: www.elsevier.com/locate/radphyschem
Mechanism of antioxidant interaction on polymer oxidation by thermal and radiation ageing$ Tadao Seguchi b,n, Kiyotoshi Tamura b, Akihiko Shimada a, Masaki Sugimoto a, Hisaaki Kudoh c a
Japan Atomic Energy Agency (JAEA), Takasaki Applied Quantum Chemistry Institute, Takasaki 370-1292, Japan JAEA, Atomic Science Institute, Tokai 319-1195, Japan c The University of Tokyo, Japan b
H I G H L I G H T S c c c c c
Interaction of antioxidant on polymer oxidation is discussed for thermal and radiation ageings. Antioxidant is very effective for thermal oxidation, but not for radiation induced oxidation. Interaction of antioxidant is not the termination reaction of radicals on polymers. Antioxidant is supposed to reduce the provability of polymer radical formation by thermal activation. Mechanism of polymer oxidation may not be chain reaction via peroxy radical and hydro-peroxide.
a r t i c l e i n f o
abstract
Article history: Received 9 May 2012 Accepted 18 June 2012 Available online 3 July 2012
The mechanism of polymer oxidation by radiation and thermal ageing was investigated for the life evaluation of cables installed in radiation environments. The antioxidant as a stabilizer was very effective for thermal oxidation with a small content in polymers, but was not effective for radiation oxidation. The ionizing radiation induced the oxidation to result in chain scission even at low temperature, because the free radicals were produced and the antioxidant could not stop the oxidation of radicals with the chain scission. A new mechanism of antioxidant effect for polymer oxidation was proposed. The effect of antioxidant was not the termination of free radicals in polymer chains such as peroxy radicals, but was the depression of initial radical formation in polymer chains by thermal activation. The antioxidant molecule was assumed to delocalize the activated energy in polymer chains by the Boltzmann statics (distribution) to result in decrease in the probability of radical formation at a given temperature. The interaction distance (delocalization volume) by one antioxidant molecule was estimated to be 5–10 nm by the radius of sphere in polymer matrix, though the value would depend on the chemical structure of antioxidant. & 2012 Elsevier Ltd. All rights reserved.
Keywords: Mechanism Degradation Polymer Oxidation Antioxidant Radiation
1. Introduction Antioxidants are essential stabilizers for most of the polymer materials. The polymer degradation is significantly reduced by mixing a small content of antioxidant as is well known. For the lifetime extension of polymer materials, many types of antioxidants have been developed for their application in various environments. However, the information of specific antioxidants (name and the content) in the practical polymer materials has been the knowhow of the processing company, because, the $ The research was under the contract between NISA (Nuclear and Industrial Safety Agency, Japan) and JAEA. n Corresponding author. E-mail address: [email protected] (T. Seguchi).
0969-806X/$ - see front matter & 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.radphyschem.2012.06.011
information of formulation (compositions of stabilizers and fillers) are the key points to determine the properties and life time of materials. The antioxidant is a miner content in polymer formulation; it is about 0.5–1.0 phr (parts per hundred resin) for crystalline polymers, and 1.0–2.0 phr for rubbers in the practical polymer materials. For most of the polymers as raw materials, a small content of stabilizer (around 0.05 phr of antioxidant) was mixed immediately after the polymerization, because without stabilizer the oxidation of polymer proceeds in air even at ambient temperature. The antioxidants are organic compounds and there are many species, which are mainly 4 groups as phenol, amine, sulfur, and phosphine (Haruna, 2010). From vast studies the oxidation schema have been established to be the chain reaction via peroxy radical and hydro-peroxide, where the role of antioxidant for polymers was the termination of
1748
T. Seguchi et al. / Radiation Physics and Chemistry 81 (2012) 1747–1751
free radicals or decomposition of hydro-peroxides in polymer chains under oxidation process (Conley, 1970; Osawa, 1992). In above schema the decrease of antioxidant content in polymer matrix during oxidation was thought to be due to the consumption in the reaction with radicals. In the studies of cable degradation for nuclear power plants by thermal and radiation ageing, the effect of antioxidant was found to be significantly different between thermal and radiation oxidations (Seguchi et al., 2011). A small amount of antioxidant was very effective for thermal oxidation, but not for radiation oxidation. The decrease of antioxidant content was supposed to be due to the evaporation in thermal oxidation, and to the decomposition in radiation oxidation. For both oxidations the primal oxidation products were the carboxylic acid formed by polymer main chain scission (Sugimoto et al., 2012). These findings indicated that the oxidation mechanisms were different from the chain reaction via peroxy radical and hydro-peroxide, and also the role of antioxidant was completely different from those in the traditional reaction schemas. In this paper, the tensile elongation behavior of crosslinked polyethylene (XLPE) containing different antioxidants was compared by thermal and radiation oxidations. The role of antioxidant in polymer oxidation was discussed, and a new schema of antioxidant interaction in polymer matrix was proposed.
2. Materials and methods The polymer materials were the crosslinked polyethylene (XLPE) containing a specific antioxidant with different contents. XLPE-A was supplied from a cable company A, and the antioxidant was 2-mercapto-benzo-imidazole (Nocrac-MB). The antioxidant content was 0.0 (no mixing), 0.1, and 1.0 phr (part per hundred resin). XLPE-B was supplied from a cable company B, and the antioxidant was 4,4-Thio-bis(3-methyl-6-t-butyl phenol) (Nocrac-300). The content was the same with XLPE-A. Both XLPEA and -B were 1 mm thick sheet specimens by chemical crosslinking after mixing the respective antioxidant. The chemical structure of antioxidants for XLPE-A and -B is shown in Fig. 1. The chemical crosslinking and the stabilizers for both sheets were the same with practical cable insulation except the antioxidant. As a reference of practical formulation of XLPE, a raw polyethylene was examined. The raw polyethylene was of high molecular weight and contained a small amount of stabilizer, but the content and type were not revealed by the polymer company as usual. The specimens were 1 mm thick sheet by hot pressing.
Fig. 1. Molecular structure of antioxidant mixed in XLPE-A (Nocrac-MB) and XLPE-B (Nocrac-300).
The oxidation of the sheet specimens was carried out by radiation ageing and by thermal ageing in air. For radiation ageing, the specimens were irradiated by Co-60 gamma rays in air at 100 1C with a dose rate of 1.0 kGy/h using a thermostat oven. Here, the irradiation was carried out at 100 1C with a dose rate of 1 kGy/h in order to spread the radiation induced oxidation throughout the XLPE sheet of 1 mm thickness. At 100 1C the rate of thermal oxidation is very small, so most of it is induced by radiation oxidation. For the thermal ageing, the specimens were stored at 100, 135, and 155 1C in air using a gear type oven. For the raw PE sheets, the thermal ageing was carried out at 95, 110, and 125 1C in air using same oven. The changes of material properties were measured by tensile test at room temperature. The sheet specimens were cut to dumbbell shape test pieces (Japan Industrial Standard 3-go; 100 mm length, 5 mm width at central part). The data were the average of 5 pieces except the abnormal data, and the reproducibility of 5 pieces was 710% for the elongation at break determined by photo-trace of the marker on test piece.
3. Results The thermal degradation, the decay of elongation at break, of raw polyethylene sheet is shown in Fig. 2. The change of the tensile strength was similar to that of elongation. A small amount of antioxidant was mixed in this polymer, so the oxidation during sheet processing above melting temperature was reduced. The oxidation proceeded by thermal ageing at 90 1C, and the elongation decreased rapidly over 100 h ageing time, and the time decreased with the increase of temperature from 110 to 125 1C. The rapid decay of elongation in Fig. 2 reflected the decrease of antioxidant in polymer matrix. Fig. 3 shows the effect of antioxidant content for XLPE-A (Nocrac-MB) degradation by thermal and radiation ageings. The initial values of XLPE-A (10 h or 10 kGy in Fig. 3) were a little different with the antioxidant content, which depended on the crosslinking density. The decays of elongation by thermal ageing at 135 1C were significantly different with the antioxidant (Nocrac-MB) content. With 0.1 phr content, the decay time extended to 10 times more than that of no Nocrac-MB (0.0 phr), but XLPE-A (0.0 phr) contained a small amount of antioxidant in the raw polymer and also the other stabilizers. By increase of antioxidant to 1.0 phr, the decay of elongation shifted to the right
Fig. 2. Elongation at break for raw polyethylene sheet by thermal ageing in air at 95, 110, and 125 1C.
T. Seguchi et al. / Radiation Physics and Chemistry 81 (2012) 1747–1751
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Fig. 3. Elongation at break of XLPE-A (Nocrac-MB: 0.0, 0.1, 1.0 phr) by thermal ageing at 35 1C and by radiation ageing.
N300 N3001.0phr 1.0 phr
N3001.0phr 1.0 phr N300
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N300 N3000.1phr 0.1 phr
N3000.1phr 0.1 phr N300
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Fig. 4. Elongation at break of XLPE-B (Nocrac-300: 0.0, 0.1, 1.0 phr) by thermal ageing at 135 1C and by radiation ageing.
(longer ageing time), and was estimated by the dotted line to over 1000 h. In the case of radiation ageing, the decay of elongation was scarcely affected by the content of 0.1 phr. However, the decay of XLPE-A with 1.0 phr shifted to high dose by 4–5 times. The shift can be explained by the anti-rad effect of aromatic compound NocracMB, and not by the antioxidant effect, because the anti-rad effect increases proportionally with the aromatic content in polymers. The decay of elongation for XLPE-B (Nocrac-300) is shown in Fig. 4, whose data were the same ones in our previous paper (Seguchi, 2011). The dotted lines for 0.1 and 1.0 phr at the longer ageing time are the estimated decay curves by considering the decrease of antioxidant. The initial value of elongation depended on the crosslinking density like that of XLPE-A. The fact that the decay of elongation for Nocrac-300 is longer than that for NocracMB indicates that the evaporation of Nocrac-300 is lesser than that of Nocrac MB at 135 1C. The decays of XLPE-B with 0.1 phr and 1.0 phr by thermal ageing were extended to longer time than those of XLPE-A with Nocrac-MB. For the radiation ageing, the
effect of 0.1 phr Nocrac-300 was small, and the effect increased with 1.0 phr as well as Nocrac-MB. In Figs. 3 and 4, the elongation tends to increase with ageing time until the rapid decay for both thermal and radiation ageings, which reflects the break of network structure of XLPE by the chain scission even at the stage of shorter ageing time for both ageings. Figs. 5 and 6 show the degradation of flame retardant ethylene-propylene rubber (EPR) insulator in FR-PH-2.0-C cable, and silicone rubber (SiR) insulator in KGB-2.0-B cable by thermal and radiation ageings for very long period at relatively low temperature and low dose rate irradiation. The data were obtained by the research group of Japan Nuclear Energy Safety Organization (JNES) through the national project for assessment of cable ageing (Yamamoto and Minagawa,2009). Both insulators were materials for the cables of a nuclear power plant. In the thermal ageing for EPR, the decay curves of elongation were similar for the different temperatures, and the decay times shifted with temperature. For radiation ageing, the decay curves are also similar for the different dose rates; here, the major
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Fig. 5. Elongation at break of frame retardant EPR (FR-EPR) insulator in FR-PH-2.0 cable by thermal ageing at 100, 110, and 120 1C, and by radiation ageing at 80 1C with dose rates 18 and 100 Gy/h in air (plots of JNES data; Yamamoto and Minagawa, 2009).
SiR-B
135 0C
600
155 0C 3 Gy/h, 100 0C
Elongation at break/%
500
18 Gy/h, 100 0C 100Gy/h, 100 0C
400
300
200
100
0 100
1000
10000
100000
Ageing time/h Fig. 6. Elongation at break of silicone rubber (SiR) insulator in KGB-2.0-B cable by thermal ageing at 100, 110, and 120 1C, and by radiation ageing at 80 1C with dose rates 18 and 100 Gy/h in air (plots of JNES data; Yamamoto and Minagawa, 2009).
degradation should be induced by radiation oxidation, because the thermal oxidation rate at 80 1C would be very small. Fig. 5 shows that the decay behavior was significantly different between thermal and radiation ageings for the practical polymer material. For XLPE insulation materials in various cables, a similar behavior with EPR in Fig. 5 was observed by the analysis of JNES data. The difference between thermal and radiation oxidation should be closely related to the content of antioxidant as well as those in Figs. 3 and 4. For SiR degradation in Fig. 5, the degradation behavior was the same between thermal and radiation ageings. The oxidation mechanism of SiR was different from that of the olefin polymers (EPR, XLPE), and the antioxidant was not necessary for SiR insulation material. Therefore, for polymers without concerning antioxidant the oxidation mechanism would be the same or similar between thermal and radiation ageings.
4. Discussion As seen in Figs. 2 and 3, a small amount of antioxidant was very effective for the XLPE degradation by thermal ageing. Even
for raw PE, a small amount of stabilizer was effective until about 100 h at a lower temperature of 95 1C. Generally the mechanical degradation as elongation at break depends on the degree of oxidation, but not proportional to the oxidation. The extent of oxidation would be proportional to the ageing time as far as the antioxidant remains sufficient in the polymer matrix. Otherwise, the oxidation of polymer materials should proceed by thermal ageing even if antioxidant remains enough content. Any antioxidant (organic compound) is released from the polymer matrix by evaporation during thermal ageing and the release rate depends on the ageing temperature. When the antioxidant content was decreased to a certain value, the oxidation rate increased rapidly with decreasing content below a limited content. The ageing time at the sharp decay of elongation in Figs. 2, 3, 4, and 5 indicated that the antioxidant content was decreased to less than the limited content. The time of the limited content depends on the initial content and molecular characteristics of the antioxidant such as molecular weight and compatibility with polymer matrix. For the practical materials containing various stabilizers and fillers, the rapid degradation due to the decrease of antioxidant content might be lessened by comparing with a simple formulation such as XLPE-A and XLPE-B. For radiation ageing at relatively low temperature, that is, the thermal oxidation is small, and the radiation induced oxidation should be proportional to ageing time (or dose). Antioxidants were essentially not effective for radiation induced oxidation. However, for XLPE with high content of antioxidant, the degradation time shifted to higher doses, which could be explained by the effect of radiation protection by aromatics in antioxidant. The evidence was observed in EPR degradation with various contents of antioxidant though the explanation was different (Baccaro et al., 1993). It was confirmed that the radiation induced oxidation is proportional to dose by the yield of oxidative products in polymers materials (Seguchi et al., 2011; Baccaro et al., 1993). So, when the oxidation is proportional to ageing time, the elongation change should show the same curves for the radiation ageing in Figs. 2–5. For example in SiR degradation, the thermal oxidation would be proportional to ageing time, so the decay curves are the same with radiation oxidation. The fact that the antioxidant has no effects on radiation induced oxidation means that the free radicals formed in polymer chains do not react with the antioxidant. Because the primal active sites were the free radicals at ambient or low temperature in the studies of radiation effects of polymers, and in the presence of oxygen, the radicals reacted with oxygen to result in chain scission and oxidative products even at room temperature (Arakawa et al., 1982). In the thermal oxidation, a small amount of antioxidant is very effective to reduce the oxidation and increase the efficiency with an excess of antioxidant, but sufficient oxidation proceeds to a certain extent with any type of antioxidant with sufficient content. Below the limited content of antioxidant, the rate of oxidation is accelerated with a decrease in the content. Therefore, the idea that the role of antioxidant is the termination of free radicals is not acceptable even in the thermal oxidation. A new concept of antioxidant interaction with polymer chains is the delocalization of activated energy in polymer matrix in thermal ageing. A model of the concept is illustrated in Fig. 7. One antioxidant molecule interacts with polymer molecules within a sphere of radius R wherein radical formation by thermal activation is reduced. The population of activated sites in polymer matrix is determined by the Boltzmann statistics, and a part of the activated sites would convert to radicals. The role of antioxidant is supposed to decrease the conversion from activated sites to radicals by delocalization of activated energy.
T. Seguchi et al. / Radiation Physics and Chemistry 81 (2012) 1747–1751
Antioxidant depress radical production in effective volume of polymer matrix by energy delocalization for thermal activation Polymer matrix
X : Antioxidant molecule
X R: 7nm
: radical
Effective volume of antioxidant. Radius of sphere is about 7nm
Fig. 7. Model of antioxidant interaction with polymer in thermal oxidation.
The interaction distance, radius R of the sphere, could be estimated from the limited antioxidant content, that is, the limited content covers the interaction over the polymer matrix. In the case of XLPE-B, 0.1 phr antioxidant was sufficient, and the limited content (at sharp decay of elongation) was estimated to be 0.02 phr. As the XLPE has 50% crystals and antioxidant can be mixed in amorphous part, the actual content in amorphous part is 0.04 phr for the limited content. The concentration of Nocrac-300 at 0.04 phr is 1.0 10 6 mol/cm3, and one molecule per 1.6 10 18 cm3 of amorphous polymer matrix. If the volume of interaction area is a sphere, the radius R is 7 10 9 m (7 nm). Though the radius may be different among the types of antioxidant, it might be supposed to be around 5–10 nm for various antioxidant. The content of stabilizer in raw polymers is around 0.05 phr of the rather low molecular weight antioxidant, which reduces the oxidation at ambient temperature as is well known. The radicals should be induced in the interaction volume with a certain probability by thermal ageing, and the radicals should react with oxygen even if enough amount of antioxidant is present. Therefore, any antioxidant could not completely stop the thermal oxidation. When the antioxidant content is lower than the limited value, the oxidation is accelerated in the outside of interaction volume. For radiation ageing, the radicals are induced in polymer chains by ionizing radiation of high energy (10–20 eV) at any temperature, and the radical yield is proportional to dose. The radicals react with oxygen without the effects of antioxidant. Therefore, the antioxidant does not affect the radiation oxidation. When the antioxidant has the aromatic compound in molecular structure, the radical yield might be reduced due to the radiation protection effect by aromatics.
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5. Conclusion On the cable degradation by accelerated thermal and radiation ageings, the effects of antioxidant to XLPE for the insulation material were investigated by tensile test. The antioxidant reduced extensively the thermal oxidation with a small content of 0.1 phr. But for radiation ageing, the effects of antioxidant were essentially nothing. The decay curves of elongation were different between thermal and radiation oxidations, which reflect the effect of antioxidant. Namely, the radiation oxidation is proportional to ageing time (or dose), but the thermal oxidation is accelerated when the antioxidant content decreased to less than a limited content. The decrease of antioxidant was induced by evaporation during thermal ageing, especially at higher temperature. Although the decrease occurred by radiation ageing, the oxidation was not affected by the decrease of antioxidant, because of no effect of antioxidant on radiation oxidation. Of course, apparently the oxidation should be accelerated during radiation ageing when the contribution of thermal oxidation increased during irradiation by the radiation decomposition of antioxidant. Based on the experimental facts and our previous data analysis, a new schema of oxidation of polymers, especially, the interaction mechanism of antioxidant was proposed. The essence of antioxidant effect was the reduction of radical yield as the oxidation initiator in polymer chains for thermal oxidation, and not to terminate the oxidation by the reaction with radicals. It was supposed that one molecule of antioxidant interacts with polymer chains in a sphere volume within 5–10 nm radius. The effect of a small amount of antioxidant for thermal oxidation could be explained by this model.
References Arakawa, K, Seguchi, T, Watanabe, Y, Hayakawa, N, 1982. Radiation-induced oxidation of polyethylene, ethylene–butene copolymer, and ethylene– propylene copolymer. J. Polym. Sci., Polym. Chem. Ed. 20, 2681–2692. Baccaro, S, Buontempo, U, D’atanasio, P, 1993. Radiation induced degradation of EPR by IR oxidation profiling. Radiat. Phys. Chem 42, 211–214. Conley, R.T, 1970. Thermal Stability of Polymers. Marcel Dekker. Haruna, T, 2010. Handbook of Additives for Polymer. CMC Publishing Co., Ltd. (Japanese). Osawa, Z, 1992. Degradation and Stabilization of Polymers. Musashino Kurieito, Co., Ltd. (Japanese). Seguchi, T, Tamura, K, Ohshima, T, Shimada, A, Kudoh, H, 2011. Degradation mechanisms of cable insulation materials during radiation–thermal ageing in radiation environment. Radiat. Phys. Chem 80, 268–273. Sugimoto, M., Shimada, A., Yoshikawa, M., Kudoh, H., Tamura K., Seguchi, T. Products analysis of polyethylene degradation by radiation and thermal ageing, in preparation. Yamamoto, T., Minagawa, T. 2009. The Final Report of Assessment of Cable Ageing for Nuclear Power Plants. JNES SS Report, JNES-SS-0903, available at: /http:// www.jnes.go.jp/english/katsudou/topics/activity_reports.htmlS.