Assessment of Radiation Damage to a Halogen-Free Cable Jacket

Chapter 32

Assessment of Radiation Damage to a Halogen-Free Cable Jacket 1

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M. Tavlet , H. Schönbacher , R. Cameron , and C. G. Richardson Downloaded by UNIV OF CALIFORNIA SAN DIEGO on January 22, 2016 | http://pubs.acs.org Publication Date: November 12, 1991 | doi: 10.1021/bk-1991-0475.ch032

1

European Organization for Nuclear Research (TIS), 1211 Geneva 23, Switzerland BP Chemicals, 1217 Meyrin 2, Switzerland 2

The EVA-based compound (BP D 2983) supplied by BP Chemicals is used for many control cable sheaths installed in CERN's Large Electron-Positron Collider (LEP). The present paper reports the effects of ionizing radiation on the mechanical properties of this flame-retardant cable jacketing material. The samples were taken from different development plaques, as well as from prototype and production cables. In addition to mechanical tests (IEC Standard 544), special tests and analyses were carried out to investigate antioxidant and filler contents in extruded materials. The results presented clearly show the difference between accelerated and long-term tests as well as between prototype and production cables. The most important influence of thefilleron the properties of the material seems to be the poor homogeneity. The results confirm also that antioxidant does not play an important role during high dose rate irradiations, but is efficient during long-term irradiations. In 1980, a safety instruction of the European Organization for Nuclear Research (CERN) implemented the use of halogen-free materials for cable-insulating and sheathing materials, for the reasons explained hereafter. In addition to the usual electrical, mechanical, thermal, and environmental endurance properties of these materials, there were also the following requirements: i) they must be flame retardant; ii) in the event offire,the resultant gases must be non-toxic and non-corrosive; iii) the smoke emittance must be low. As a result of these requirements, some commonly used materials had to be excluded : e.g. PVC, Hypalon, Neoprene, fluorocarbons and other halogenated and sulphur containing compounds. It was further required that the materials be radiation resistant, and in the specification for the Large Electron- Positron Collider (LEP) an elongation-at-break that retains more than 50% of its initial value after an irradiation of 5xl0 Gy at high dose rate was stipulated. For the last 10 years, since the introduction of these halogenfree materials, more than 300 cable compounds have been tested at CERN (2). The replacement of the halogen-containing materials was achieved in two steps: EPR/EPDM-based rubber compounds were used for medium- and high-voltage power cables, and polyolefin-based compounds were used for control and lowvoltage cables. 5

0097-6156/91/0475-0509S06.00/0 © 1991 American Chemical Society

In Radiation Effects on Polymers; Clough, Roger L., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1991.

Downloaded by UNIV OF CALIFORNIA SAN DIEGO on January 22, 2016 | http://pubs.acs.org Publication Date: November 12, 1991 | doi: 10.1021/bk-1991-0475.ch032

510

RADIATION EFFECTS ON POLYMERS

The main supplier of control cables for LEP, was a Norwegian cable manufacturer, who used an EVA-based material (BPD 2983) supplied by BP Chemicals. This material was tested on plaques and was approved. Also, the sheath of a prototype black cable was tested and found to conform to CERN specifications. Samples were subsequently taken from production cables and tested. These results showed a drastic decrease in the radiation resistance of the sheathing material. The tests were repeated and the susceptibility to radiation was confirmed. In order to find the reasons for this, special tests and analyses were made to check the antioxidant and filler contents in extruded materials. The present paper reports the studies of the effects of ionizing radiation on the mechanical properties of the EVA jacket material taken from different development plaques, as well as from prototype and production cables. Experimental Materials and Test Samples. The base material is an EVA copolymer, produced from a mixture of ethylene and vinyl acetate (28%). The polymeric chains are similiar to those of polyethylene, with acetate side groups [CH -C(0)-0-] and also random carbon side chains. The material contains aluminium trihydrate (60%), which is used as a flameretardant mineralfiller.The flame-retardancy property is rarely affected by radiation although the bond between the filler and the polymer may be modified. The antioxidant is of the unpolymerized phenolic type; the typical content is 0.2%. It is from two different suppliers with two trade names. The base molecule is shown here below. With its two substituted aromatic groups, it acts as a radicalscavenger and as an energy-scavenger, which neutralizes radicals and excited sites produced in the polymeric chain. 3

Antioxidant;

CH

Santonox or TBM/60

The material of the prototype cable as well as that of a further plaque contains a carbon black batch (2.5%) present both as colorant and as UV stabilizer. Its presence seems to have only a minor influence on the radiation resistance of the material. Thefirstmoulded plaques of this material were received by CERN for testing in 1982 (each sample was given a number e.g. CERN 652). After approval by CERN, the material was used by the Norwegian cable manufacturer to produce cable sheaths. Thefirstprototype cable supplied for the LEP project was black (CERN 766). Later, tests were carried out on other moulded plaques supplied by BP (CERN 885) and on many production cables supplied by the cable factory. Table I presents most of the tested materials and their relevant characteristics. In an attempt to explain the observed discrepancies in the test results, tensile samples were taken from different parts of one cable. After production, the cables are rolled on drums, and they can retain their curved shape for several weeks or months until installation. A permanent deformation may therefore take place. Samples were taken from the outside of the curved cable (CERN 930 O), from the side of the curve (CERN 930 S), and from the inside (CERN 9301).



32. TAVLETETAL

511 Radiation Damage to a Halogen-Free Cable Jacket

In order to check whether post-irradiation oxidative degradation takes place, two sets of 10 samples were taken from the same sheath (CERN 942), and were irradiated at the sametimeand under identical conditions. The first set was tested just after irradiation; the second set was tested three months later. Three other materials (CERN 943, 944, and 945) were also taken from production cables. To improve the statistics, ten samples were cut for each dose and dose rate. Finally, more plaques were prepared with different amounts of antioxidant (and with the same nominal content of mineral filler) (CERN 946 to 950). They were irradiated in the reactor and in the cobalt source. The influence of the antioxidant on the mechanical properties of the irradiated plaques was then evaluated. Table I : List of some tested materials (Cable type gives the number of conductors. The irradiation conditions are explained hereafter) CERN No Test samples (type)

Date

Thickness Irradiation (mm) conditions

652 885

BP original plaque BP plaque

1982 2.00 + 0.05 1987 2.00 ±0.05

R R

L L

766 860

Prototype cable (black) Production cable NF-48

1985 1.50 + 0.1 1986 1.85 ±0.1

R R

L L

894 895

Production cable NE-48 Production cable NE-2

IX- 1986 1.50 + 0.1 I- 1986 1.40 ±0.2

909 910 911 912

Production cable NH-7 Production cable NF-12 Production cable NE-18 Production cable NE-18

XI-1986 rv- 1987 I- 1986 IX- 1986

930 942

Production cable (0,I,S) Production cable NH-25

1985 1.60 + 0.05 1986 1.50 ±0.05

943 944 945

Production cable NE-48 Production cable NH-48 Production cable NF-48

I- 1986 1.12 + 0.07 XI- 1986 1.30 + 0.1 Π- 1987 1.50 ±0.1

946 947 948 949 950

Plaque without antioxidant Plaque with 0.1 % antioxidant Plaque with 0.2 % antioxidant Plaque with 0.3 % antioxidant Plaque 0.2% antiox.+ carbon

1987 1987 1987 1987 1987

1.30 + 0.25 1.45 + 0.1 1.10 + 0.15 1.40 ±0.1

1.5 +0.05 1.5 +0.05 1.5 +0.05 1.5 +0.05 1.5 +0.05

RC RC R C R C RC RC C C L R CC R CC R CC R C R C RC RC R C

Irradiation Conditions. Three irradiation sources were used: i) The ASTRA reactor: 95% of the dose absorbed by the plastic materials comes from gamma-rays, with an energy between 1 and 2 MeV; the remaining 5% comes from neutrons. The dose rate in this reactor is between 7xl0 and 2.7xl0 Gy/h. The maximum temperature of the samples is 62°C.; the irradiation medium is air. This temperature is low enough and thetimeof irradiation so short that we do not need to take the temperature-degradation effect into consideration. 4

5


512


ii) A ^Co source with a dose rate between 3 and 4.5 kGy/h. This irradiation condition was considered as an alternative to the reactor. According to the IEC 544 Standards (2), it is at the lower end of the 'high dose-rate' range (> 1 Gy/s). The samples are irradiated in air at ambiant temperature. iii) A ^Co source with a dose rate between 70 and 100 Gy/h. This source is used for the assessment of the long-term ageing effects, as recommended by IEC 544. These three sources are marked respectively 'R', 'C., and 'L' in Table I. A fourth source, which is also a 6°Co source, with a dose rate of 11,500 Gy/h, was used only for the three materials CERN 943,944, and 945. In our routine programme, samples are irradiated at 2xl0 , 5xl0 and lxlO Gy, plus occasionally 5xl0 Gy in the reactor. For the long-term irradiations, only the two lower doses are retained, plus a lower dose for some materials. The exact doses and dose rates are given in the individual tables of results.


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6

Radiation Effects. The free radicals with unpaired spins, and the loosely bound trapped electrons, produce optical coloration of the irradiated polymers. In addition, the unsaturations resulting from hydrogen evolution or from other decomposition may yield conjugated groups with electron excitation levels in the visible spectral range. For the material under investigation, the modifications of the optical properties were not important, just a small darkening was observed after irradiation. During irradiation, the electrons that are excited to the conduction band, and the hydrogen radicals, increase the conductivity of the irradiated polymer. This phenomenon is limited to a short period after irradiation. Permanent changes of the electrical resistivity of irradiated polymers can be produced by cracks, bubbles, and similar flaws that result from radiation damage. As permanent changes of the electrical properties result from the degradation of the mechanical properties, it is consistent to test the latter in order to estimate the degradation of a cable insulation material (2). Gas evolution in the irradiated polymer may lead to bubble formation, which initiates cracks under stress. This occurs only when irradiating with high-LET particles at high dose rates. In low-density polyethylene and in EVA, the two mechanisms of chain scission and cross-linking compete. In general, it seems that for low irradiation doses, the cross-linking occurs first, and that chain scission predominates for higher doses. Degradation is promoted during long-term irradiation in the presence of oxygen (3). Test Methods. Following the recommendations of the International Electrotechnical Commission (2), the materials were submitted to tensile tests. These tests were carried out on an Instron 1026 machine. Elongation-at-break and tensile strength were registered. Dumb-bell samples (normalized S2 according to ISO/R 37/2) were cut from the plaques or from the extruded sheaths. The mechanical tests were carried out on both irradiated and non-irradiated samples. The hardness Shore D was also measured, using a Volpert apparatus. Furthermore, irradiated and non-irradiated samples were submitted to ESR spectroscopy in a Varian E3 spectrometer. From previous studies (4) we find that in this kind of material the ESR signal comes mainly from the irradiated aluminium trihydrate. Therefore the height of the signal at a given dose (and a given sample mass) is assumed to be proportional to the filler content. The antioxidant content in the samples was checked by the technique of UV absorption (at 280 nm) and also by solvent extraction. Results The detailed results are given in the form of graphs plus tables for materials 652,766, 860, 930, and 942 (Figures 1 to 5). For the other materials, some results appear in the tables shown below. In Radiation Effects on Polymers; Clough, Roger L., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1991.


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BP Chemicals Plates 1982

REMARKS

Dose

100.0

0.500

1.2 1.7

1.2

5.3 ± 0.4

6.6 ± 0.6

8.0 ±

10.3 ± 0.3

8.2 ± 0.2

8.0 ± 10.6 ±

R (MPa)

1.4

Gy/h ) : 5.8 Gy/h ) : 5.4

58.5 ± 18.7

526.5 ± 13.6

610.0 ± 5 5 . 6

19.0 ±

160.0 ± 26.2

377.5 ± 3 0 . 5

610.0 ± 5 5 . 6

Ε (%)

Elongation

Tensile properties Strength

RADIATION INDEX ( 1.7x10 RADIATION INDEX ( 100.0

_ 100.0

0.200

1.7x 1 0

0.000

5

1.7x 1 0

1.000 5.000 5

5

1.7x 1 0

0.000 0.500

(Gy/H)

rate (MGy)

Dose

TEST RESULTS :

MCA 319

SUPPLIER

652

(D 2983 FR)

Thermoplastic EVA

MATERIAL TYPE

CERN MATERIAL No.

Figure 1: Degradation of mechanical properties of material 652 with absorbed dose.

ABSORBED DOSE (Gy)

10

10

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LONG-TERM

SHORT-TERM

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35.5 ± 0.0

41.0 ± 0.0

42.0 ± 0.0

47.0 ± 0.0

43.0 ± 0.0

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42.0 ± 0.0

Η (Degree)

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RADIATION INDEX ( 1.7x10 RADIATION INDEX ( 550.0

100.0

0.500

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6.0 ±

60.0 ± 1 1 . 8

7.1 ± 0.5

0.100

4.1 ± 0.2

526.0 ± 36.1

6.2 ± 0.7

1000.0

0.000 100.0

583.3 ± 45.5

4.9 ± 0.4

5

1.7 χ 1 0

2.500

0.200

46.0 ± 0.0

36.7 ± 17.6

1.6

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31.0 ± 0.0

29.0 ± 0.0

28.0 ± 0.0

25.0 ± 0.0

49.0 ± 0.0

35.0 ± 0.0

583.3 ± 4 5 . 5 397.0 ± 35.1

6.2 ± 0.7 5

-

Shore C Η (Degree)

1.7 χ 1 0

Ε (%)

Hardness

0.500

R (MPa)

(Gy/H)

(MGy)

Elongation

0.000

Strength

Dose rate

Dose

Tensile properties

Norsk Kabelfabrlk EB Taken from LEP prototype cable

REMARKS TEST RESULTS :

BP D 2983 FR (black)

SUPPLIER

Polyolefin thermoplastic

766

TYPE

MATERIAL

CERN MATERIAL No.


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io

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4.5

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± ± ± ±

0.2 0.5

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RADIATION INDEX ( 100.0

RADIATION INDEX ( 71 10.0

RADIATION INDEX ( 1.7x10

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0.3

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5.0 5

1.7x10

0.000 0.200

R (MPa)

(Gy/H)

(MGy)

Hardness

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3.6

G y / h ) < 5.0

G y / h ) : 5.1

G y / h ) < 5.3

35.2 ± 14.5 6.4 ± 1.7

99.2 ± 53.9

464.8 ± 1 4 7 . 8

29.6 ±

287.2 ± 1 9 4 . 8 62.4 ± 46.4

31.0 ± 0.0

27.0 ± 0.0

25.0 ± 0.0

25.0 ± 0.0

32.0 ± 0.0

27.0 ± 0.0

25.0 ± 0.0

25.0 ± 0.0

35.0 ± 0.0

20.8 ±

4.1

30.0 ± 0.0

29.0 ± 0.0

25.0 ± 0.0

Η (Degree)

Shore D

37.2 ± 10.9

229.3 ± 1 9 9 . 9

464.8 ± 1 4 7 . 8

Ε (%)

Elongation

Tensile properties Strength

Dose rate

Dose

Taken from NF-48 LEP cable (C)

0.000 10'

2

io

TEST RESULTS :

REMARKS

BP D 2983 FR (white) Norsk Kabelfabrik EB

SUPPLIER

Polyolefln

860

TYPE

MATERIAL

CERN MATERIAL No.

0.100

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ABSORBED DOSE (Gy)

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Assessment of Radiation Damage to a Halogen-Free Cable Jacket

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