The influence of γ-irradiation and chemical structure on the dielectric properties of polypropylene

Radiat. Phys. Chem. Vol. 48, No. 1, pp. 129-130, 1996

Pergamon

S0969-8~X(96)0002~'4

Copyright © 1996 Published by ElsevierScienceLtd Printed in Great Britain. All rights reserved 0969-806X/96 $15.00+ 0.00

THE I N F L U E N C E OF 7-IRRADIATION A N D CHEMICAL STRUCTURE ON THE DIELECTRIC PROPERTIES OF POLYPROPYLENE H. M. B A N F O R D , l R. A. F O U R A C R E , 2 A. F A U C I T A N O , 3 A. B U T T A F A V A 3 and F. M A R T I N O T T I 3 'Scottish Universities Research and Reactor Centre, East Kilbride G75 0QU, Scotland, 2Centrc for Electrical Power Engineering, University of Strathclyde, Glasgow Gl IXW, Scotland and 3Department of General Chemistry, University of Pavia, 27100 Pavia, Italy

The premature ageing of polymers due to oxidative degradation is of concern for the integrity of electrical insulation, particularly in the presence of ionising radiation when complex chemical reactions give rise to new functional groups and structures. Films ( ~ 50/~m) of isotactic polypropylene either additive free or containing 2,2-6,6 tetramethylpiperidine derivatives (HALS) were oxidised which was achieved by irradiation in air at a dose rate of 1 k G y / h using a 6°Co y-source. Test material was left in air for a period in excess o f I yr so that in source and post irradiation oxidation occurred. Investigations of the irradiated material were undertaken using product analysis and E P R identification o f the intermediate radical species. Determination of the reaction products was by quantitative F T I R and conventional analytical methods modified to circumvent the insolubility of PP. The products created in source are indicated in Table 1. The oxidation was autocatalytic for carbonyl and ketone production while the concentration of hydroperoxides tended to saturate, indicating decomposition to other oxidative products. Thus carbonyl groups were abundant and spread over a range of species. The addition of H A L S to the polypropylene decreased the oxidation rate by a factor > 2 and was characterised by the presence of nitroxide radicals whose concentration passed through a maximum with time.

Low concentration levels in samples left for over a year post irradiation limited analysis to hydroperoxides, alcohols and ketones. The dominating reaction was hydrogen abstraction by peroxy radicals to yield hydroperoxides ( > 8 0 % of oxygen absorbed). The reaction kinetics were dispersive and the rate parameters approached a constant value (a = 0.9). The addition o f H A L S to the polypropylene produced a considerable reduction in oxidation rate and the formation o f nitroxide radicals. A kinetic simulation based on nitroxide regeneration showed that this mechanism could only be a partial explanation of the observations. Dielectric loss (DL) at very low frequency and thermally stimulated discharge currents (TSDC)

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Dose (kGy) 7.3 15 23.4 31.2 46.8 54.6 93.6 143 198.6

Alcohols (tool/L) 3.00 E-04 3.10 E-03 7.80 E-02 1.53 E-02 3.65 E-02 8.41 E-02

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Temperature(K) Fig. 1. TSDC spectrum of additive free polypropylene irradiated to 38.6 kGy with y-rays.

Table 1 Hydroperoxides (tool/L) 3.28 E-02 7.11 E-02 1.11 E-01 1.33 E-01 1.99 E-01 2.45 E-01 3.42 E-01 4.54 E-01 5.78 E-01

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Ketones (mol/L) 1.69 E-03 5.83 E-03 1.13 E-02 1.60 E-02 3.03 E-02 3.79 E-02 7.42 E-02 1.53 E-01 2.51 E-01 129

Aldehydes (tool/L)

Esters (tool/L)

Lactones (tool/L)

2.07 E-03

1.70 E-03

1.60 E-02

9.30 E-03

8.20 E-03

8.60 E-02

2.98 E-02

2.98 E-03

130

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Frequency(Hz) Fig. 2. Dielectric loss factor (Hamon approximation) of additive free polypropylene irradiated to 38.6 kGy with 7-rays. measurements were made on additive free polypropylene. This material produced insignificant TSDC and

DL spectra while material irradiated to 38.6 kGy showed TSDC peaks (Fig. 1). The corresponding DL spectra are shown in Fig. 2. The addition of HALS produced a change with characteristics generally similar to those for additive free material. However at 383 K a DL was observed in the region of 10 E3 Hz with a corresponding TSDC peak. Irradiation to 93 kGy produced a DL spectrum showing a peak shifting with temperature and a TSDC spectrum showing peaks. The signals are lower than those obtained for the additive free material and corresponds to lower levels of oxidation products. It has not proved possible to link specific functional groups with specific dielectric behaviour. Investigations have now commenced on the introduction of hydroperoxide and ketone functional groups to model the behaviour of oxidation products in polypropylene.