Influence of thermal ageing on electrical properties of ethylene- propylene elastomers ... Polymers undergo oxidative degradation by diffusion of oxygen into the ...
JOURNAL OF MATERIALS SCIENCE LETTERS 15 (1996) 871-873
Influence of thermal ageing on electrical properties of ethylenepropylene elastomers T. ZAHARESCU*, L. CARAMITU ICPE SA, 313 Splaiul Unirii, CP 10.4, Bucharest 74204, Romania E ILIESCU ARPECHIM SA, Piteti 0300, Romania
Polymeric dielectrics are widely used in electrical devices, especially saturated materials, because of their long-term stability in various conditions of operation. Fundamental requirements which can be applied to the endurance limit of any material are defined in terms of electrical, mechanical, thermal and environmental influences involved in product failure [1]. Polymers undergo oxidative degradation by diffusion of oxygen into the inner layers of the sample, and by its reactions with available entities, such as free radicals. Evaluation of ageing control is a safety question which is concerned with the prevention of accidental opportunities. Thermal oxidative ageing of ethylene-propylene copolymers at elevated temperatures [2] provides profiles of oxygenated products concentrations. New structures (alcohols, ketones, acids, etc.) can be considered as dipoles, depending on the organic function which is present. It is well known [3] that the structure of any polymer is responsible for changes of physical properties of the material. Localized polar molecules formed by chemically trapped oxygen show different dipole moments, which can explain the particular e~[ectrical character of the tested po]ymer. Evaluation of the thermal degradation of ethylene-propylene copolymers [4] and the direct consequences of further behaviour has to be considered as the starting point of material qualification. In this letter the time dependence of some electric parameters (relative dielectric constant, loss tangent, volume and surface resistivities) were determined by isothermal heating of polymer square sheets (1.0 mm thick) in air at 100 °C. The tested materials were not previously purified in order to check them in the same state as they are delivered for cable manufacture. Before electrical measurements, samples were conditioned in a dissicator fi»r 24 h at room temperature. It has been shown [5] that hydroperoxides generated by reacdon of oxygen with free radicals or other reactive units of polymers are involved in the propagation step of oxidytive degradation. At the same time, simple conversion of intermediates (peroxy radicals and hydroperoxides) into alky! radicals by elimination of oxygen takes place with *To wbom correspondence should be ad&esse& 0261-8028
© 1996 Chapman & Hall
very low probability in the presence of oxygen. Thus, the damage process of ethylene-propylene copolymers is promoted by a peroxidation reaction, followed by oxygenated function development. In solid polymers, the probability of displaying two close polar groups on the same macromolecule is very small, because it is not practically possible to react simultaneously or successively two neighbour positions with oxygen. Differences in dipole moment magnitudes for oxygen bonds [6] (O-H 1.5 D; C-O 1.2 D; C=-O 2.7 D) explain the changes in electrical properties of degraded polymers. If po]ymer samples are warmed for a long time, 4
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Figure I Changes of volume resistivity for isothermal degradation of tested elastomers at 100 °C: (0) EPR; (A) EPDM. 2 4-"
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Figure 2 Changes of surface resistivity for isothermal degradation of tested elastomers at 100 °C: (0) EPR; (A) EPDM.
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Figure 3 Changes of relative dielectric constant for isothermal degradation of tested elastomers at 100 °C: (Q) EPR; (A) EPDM; ( - - - ) f = 10 kitz; ( ) f = 1 kHz.
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105~ ~ ' " A - . the start o f d e g r a d a t i o n w i l l involve only d i s s o l v e d o x y g e n to generate o x y g e n a t e d units. A detailed m e c h a n i s m o f t h e r m a l d e g r a d a t i o n o f ethylenep r o p y l e n e elastomers can be found elsewhere [1]. W h e n material d a m a g e is carried out h y d r o p e r o x i d e s are t r a n s f o r m e d to a l d e h y d e s and ketones. This process shows that changes in electrical p r o p e r t i e s
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Figure 4 Changes of loss tangent for isothermal degradation oftested elastomers at 100 °C: (a) EPR; (b) EPDM; (----) f = 10 kHz; (----) f = 1 kHz.
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Time (s) Figure 5 Decay ofvolume resistivity dm'ingrelaxation process Ihr different degradationlevels of tested elastomers at 100 °C: (O) EPR; (40 EPDM. (a) initial; (b) 200 h; (c) 400 h; (d) 600 h; (e) 800 h; (f) 1000 h; (g) I200 h; (h) 1400 h; (i) 1600 la; (j) 1800 h; (k) 2000 h; (1) 2200 h. 872
will mirror the relative distribution of different polar products; either they are the initiators of the chain reaction or they a r e formed afterwards as final structures. Fjgs 1 and 2 show the"changes of volume and surface resistivities during thermal ageing of the tested materials in air at 100°C. Copolymers generate a lower amount of polar products after 200h of heating; their concentration will be maintained constant for the next 400 h, in spite of the" different behaviour of the corresponding terpolymer. This assumption that more polar hydroxy groups are responsable for C - O formation is reasonable, because sharp changes in resistivity values for advanced ageing describe this reaction route. This process was also noted in the thermal degradation of ethylene-propylene elastomers in salt solutions [7]. Figure 3 shows that the diei[ectric constant is smoothly changed after the exposure for 2200 h at 100°C. Moreover, a difference of one order of magnitude between the measuring frequencies did not induce a significant effect oll the alteration of dielectric constant during thermal degradation of the tested elastomers. In contrast, after long-term accumulation of polar products (after about 800 la), the loss tangent of the aged elastomers did not present monotone behaviour (Fig. 4). Simultaneous accumulation and orientation of more and more dipoles require electrical energy to order them. During electrical measurements this ordered disposal is disturbed either by further heating or by electrical interactions between the closest dipoles. Fig. 5 develops the mobility of polar macromolecules (or a part of them) after the application of an
electric field. The orientation ability of polymer segments illustrates the degradation level of the tested materials. There is a difference in the mobility of the polar part of these two types of elastomers. While the ethylene-propylene copolymer may be oxygenated at the head of its molecules or on the branches placed on the carbon chain, the chemical structure of ethylene-propylene-diene terpolymer allows oxygen binding at random places, because the added diene is more susceptible attack by oxygen. The large differences between the relaxation behaviour of these two elastomers is diminished after about 800 h of degradation, because the increased amount of oxygenated products tends to reach the same level in steady-state conditions.
References 1. C. H. FISHER, in "Handbook of polyolefins. Synthesis and properties', edited by C. Vasile (Marcel Dekker, New York, 1993) Chapter 11. 2. R.P. SINGH, R. MANI, S. SIVARAM, J. LACOSTE and J. LEMAIRE, Polym. [nt. 32 (1993) 189. 3 . D. A. SEANER, "Electrical properties of polymers" (Academic Press, New York, 1982) p. 24. 4. T. ZAHARESCU and M. GIURGINCA, Mater. Plastics (Bucharest) 30 (1993). 302. 5. J. L. BELLAND and G. GEE, Trans. Faraday Soc. 42 (1946) 236. 6. C. D. NENITZESCU, "Organic chemistry", Vol. I, 7th Edn (Bueharest, 1973) (Romanian version). 7. T. ZAHARESCU, Polymer 35 (1994) 3795.
Received 15 September and accepted 9 November 1995
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