Properties of High-Voltage Rubbers
Abstract Generated abstract
This study compares candidate rubber insulation materials for flexible high-voltage cables, where porosity and operation at fields above about 3 kV/mm can promote ionization, ozone attack, and corona deterioration. Butyl rubber, ethylene-propylene rubbers, several organosilicon rubbers, and a standard rubber were tested for volume resistivity, dielectric loss, permittivity, breakdown strength, and mechanical behavior over 20 to 120°C, including thermal aging. The results indicate that butyl rubber, ethylene-propylene rubber based on a terpolymer, and organosilicon rubber based on Silasten-50 have the best dielectric performance among the materials examined, although the terpolymer rubber shows low tensile strength and aging-related loss of elongation. The paper also finds that ethylene-propylene rubber properties can be improved by increasing hardness and using Aerosil filler, while sulfur-free butyl rubber outperforms sulfur-vulcanized stock in dielectric, mechanical, heat-resistance, and uniformity characteristics.
Full Text
PROPERTIES OF HIGH-VOLTAGE RUBBERS
M. N. TRESKINA, N. R. GONCHAR, S. F. GODUNOVA
(Presented by the scientific seminar of the Department of Electrical Insulation and Cable Engineering)
In flexible high-voltage cables rated at 6 kV and above, rubber is an irreplaceable material for insulation. Owing to structural and technological features, rubber is inherently porous. When rubber insulation operates in an electric field with an intensity of 3 kV/mm and higher, ionization occurs in gas pores, leading to deterioration of the principal properties of the rubber insulation of a flexible cable.
High-voltage rubbers, along with other requirements, must possess high ozone and corona resistance. Increasing the ozone and corona resistance of rubbers can be achieved through the use of special rubbers and stabilizers; the most effective is the use of synthetic rubbers with low unsaturation. In terms of ozone resistance, the rubbers used in high-voltage rubbers may be arranged in decreasing order as follows: silicone-organic (saturated compound), ethylene-propylene (saturated compound), butyl rubber (unsaturation 0.6–1.0 mol%), natural rubber (unsaturation up to 96 mol%) [1]. According to [2], the corona resistance of butyl-rubber rubber is 2 times higher than that of rubbers based on natural rubber, and the corona resistance of ethylene-propylene rubber is 1000 times higher than the corona resistance of butyl-rubber rubber. Thus, as the unsaturation of the rubber decreases, the ozone and corona resistance of rubbers increases.
However, the low unsaturation of rubbers complicates the process of vulcanization of rubber compounds, necessitates the selection of special vulcanizing groups, the composition of the compound, and the choice of vulcanization conditions, which affects the properties of the rubber.
In this work, the electrical-insulation and mechanical properties of insulating rubbers based on various rubbers were investigated: butyl-rubber rubber (BK), ethylene-propylene rubbers based on SKEP (EPR) and on a terpolymer (EPTF), silicone-organic rubbers based on “Silasten-50” rubber (KO-1) and on SKT rubber (KO-2), silicone-organic rubber based on a fluorine-containing rubber (KO-F), and TSP-35A rubber. The rubber compounds were prepared according to standard formulations and vulcanized in the laboratory of TomNIIKP.
To evaluate the properties of insulating rubbers under conditions close to service conditions, tests were carried out on samples of various thicknesses (1 mm and 4–5 mm) in the temperature range from +20 to +120°C.
The resistance of rubbers to the action of a thermal field was assessed from the change in mechanical properties.
The specific volume resistivity of the rubbers was calculated from currents measured on a galvanometric setup using a tube electrometer, after a 1-minute holding of the specimen under voltage. \(\operatorname{tg}\delta\) and \(\varepsilon\) of the rubbers were determined using a P-525 bridge. Electrical strength was evaluated from the breakdown voltage of rubber specimens with needle-plane electrodes. The thickness of the layer being punctured (0.8 mm and
Fig. 1. Dependence of \(\lg \rho_v\) on temperature for rubbers at a specimen thickness of 4--5 mm
Legend:
○ — TSh-35 A;
— × — butyl-rubber compound (BK);
— ● — rubber based on ethylene-propylene rubber with Shore hardness 280 G with aerosil (EPR);
— ● — ethylene-propylene rubber based on a ternary copolymer (EPTR);
⋯ △ ⋯ organosilicon rubber based on “Silosten-50” rubber (KO-1);
— ▲ — KO rubber based on SKT rubber (KO-2);
— △ — KO rubber based on fluorine-containing rubber (KO-F).
2.5 mm) was set by moving the needle with a micrometer screw. Measurements of the mechanical properties of the rubbers were carried out according to the standard procedure.
The experimental data obtained are presented in Figs. 1–5 and in Table 1.
For all the rubbers studied, the temperature dependences of the properties \(S_v\), \(\operatorname{tg}\delta\), and \(\varepsilon\) are analogous for thin (1 mm) and thick layers (4–5 mm).
Table 1
Electrical-insulating and mechanical properties of TSP-35A rubber, butyl-rubber (BK), ethylene-propylene (EP), and organosilicon (KO) rubbers
| Characteristic | TSP-35A rubber | BK rubber | EP rubber based on SKÉP | EP rubber based on ternary copolymer | KO rubber based on “Silasten-50” | KO rubber based on SKT | KO rubber based on fluorosiloxane rubber |
|---|---|---|---|---|---|---|---|
| $\rho_v$, ohm·cm, at 20°C | $1 \cdot 10^{15}$ | $6 \cdot 10^{15}$ | $1.7 \cdot 10^{14}$ | $6.3 \cdot 10^{16}$ | $8 \cdot 10^{14}$ | $1.6 \cdot 10^{13}$ | $5.6 \cdot 10^{10}$ |
| $\rho_v$, ohm·cm, at 100°C | $3.2 \cdot 10^{12}$ | $1.8 \cdot 10^{13}$ | $8 \cdot 10^{11}$ | $8 \cdot 10^{12}$ | $5 \cdot 10^{13}$ | $1 \cdot 10^{12}$ | $<10^7$ |
| $\rho_v$, ohm·cm, multiplicity of decrease | $3.1 \cdot 10^2$ | $3.3 \cdot 10^2$ | $2.1 \cdot 10^2$ | $8 \cdot 10^3$ | 16 | 16 | $>10^3$ |
| $\operatorname{tg}\delta$ at 20°C | 0.004 | 0.010 | 0.006 | 0.007 | 0.003 | 0.130 | 0.150 |
| $\operatorname{tg}\delta$ at 100°C | 0.060 | 0.026 | 0.026 | 0.024 | 0.030 | 0.150 | 60°C—0.99 |
| $\operatorname{tg}\delta$, multiplicity of increase | 15 | 2.6 | 4.3 | 3.4 | 10 | 1.15 | 6.6 |
| $\varepsilon$ at 20°C | 3.77 | 2.37 | 3.60 | 3.54 | 3.0 | 10.0 | 6.6 |
| $\varepsilon$ at 100°C | 3.87 | 2.58 | 3.52 | 3.50 | 2.8 | 10.5 | 60°C—14.0 |
| $\varepsilon$, % change | +2.7 | +8.9 | −2.2 | −1.1 | −6.7 | +5 | +112 |
| $U_{\mathrm{br,\,avg}}$ with layer thickness 2.5 mm, kV | 36.8 | 38.0 | 36.2 | 37.3 | 39.8 | 27.3 | 25.4 |
| Mean-square deviation, kV | 3.74 | 3.75 | 2.99 | 3.38 | 4.37 | 2.22 | 2.74 |
Continuation of Table 1
| 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 |
|---|---|---|---|---|---|---|---|
| Tensile strength, kg/cm² before aging | 75.0 | 48.4 | 25.9 | 34.8 | 52.9 | 25.7 | 63.0 |
| Tensile strength, kg/cm² after 4 days of aging at 120°C | 51.3 | 47.0 | 31.2 | 34.3 | 53.8 | 29.4 | 61.0 |
| Tensile strength, kg/cm², % change | −31.6 | −2.9 | +20 | −1.4 | +1.7 | +14.4 | −3.2 |
| Relative elongation, % before aging | 486 | 668 | 683 | 572 | 426 | 463 | 467 |
| Relative elongation, % after 4 days of aging | 446 | 622 | 711 | 268 | 401 | 458 | 364 |
| Relative elongation, %, % change | −8.2 | −6.9 | +4.1 | −53 | −5.9 | −1.1 | −22 |
| Residual elongation, % before aging | 44 | 52 | 29 | 19 | 0 | 5 | 23 |
| Residual elongation, % after 4 days of aging | 44 | 49 | 35 | 0 | 0 | 4 | 17 |
| Residual elongation, %, % change | 0 | −5.8 | +20 | −100 | — | −20 | −26 |
The breakdown strength of rubber specimens with a layer thickness of 2.5 mm is 40–50% lower than the breakdown strength of specimens with a layer thickness of 0.8 mm.
At a temperature of 20°C, the highest value of $\rho_v$ is exhibited by EPTR rubber ($\rho_v = 6.3 \cdot 10^{16}$ ohm·cm), and the lowest $\rho_v$ by KO-F rubber ($\rho_v = 5.6 \cdot 10^{10}$ ohm·cm) (Fig. 1). When the temperature is raised to 100°C, a considerable decrease in $\rho_v$ is observed for EPTR rubber (by a factor of $8 \cdot 10^3$) and an insignificant decrease for KO-1 and KO-2 rubbers (by a factor of 16) (Table 1).
At 20°C the rubbers studied have close values of $\operatorname{tg}\delta$, amounting to 0.003–0.010 (Fig. 2). The exceptions are silicone-
Fig. 2. Dependence of $\operatorname{tg}\delta$ on temperature for rubbers at a specimen thickness of 4–5 mm. Designations—see Fig. 1.
organic rubbers, designated above as KO-2 and KO-F, whose $\operatorname{tg}\delta$ is equal to 0.12–0.15. When the temperature is raised to 100°C, $\operatorname{tg}\delta$ for TSSh-35A, KO-1, and KO-F rubbers increases by a factor of 10–15; for the remaining rubbers the increase in $\operatorname{tg}\delta$ is 2–4 times. The dielectric permittivity of BK rubber is 2.37; $\varepsilon$ for the silicone rubbers KO-F and KO-2 is 6.5 and 10, respectively; for the remaining rubbers $\varepsilon = 3$–3.8. With increasing temperature, $\varepsilon$ for EPR, EPTR, and KO-1 rubbers decreases, while $\varepsilon$ for the remaining rubbers increases (Fig. 3).
The best rubber in terms of electrical strength is KO-1, and the worst are KO-2 and KO-F (Fig. 4).
A comparison of the electrical-insulating properties of the rubbers studied shows that silicone-organic rubbers based on fluorine-containing
rubber and SKT have significant electrical conductivity, high \(\tan\delta\) and \(\varepsilon\), and consequently high polarity and a low value of electric strength. In addition, rubber with SKT is inferior to the others in mechanical strength.
In terms of electrical-insulating properties, the best are butyl-rubber, ethylene-propylene rubber based on a terpolymer,
Fig. 3. Dependence of \(\varepsilon\) on temperature for rubbers at a specimen thickness of 4–5 mm. Designations—see Fig. 1.
Fig. 4. Integral probability curves of breakdown of rubber specimens 4–5 mm thick under alternating-current voltage, with needle–plane electrodes and a punctured-layer thickness of 2.5 mm. Designations—see Fig. 1.
and silicone-organic rubber based on “Silasten-50”; however, their mechanical strength is respectively 36, 53, and 30% lower than the strength of the standard rubber TSh-35A. In addition, in the process of ther-
After thermal aging of EPTR rubber, a considerable decrease in relative and residual elongations is observed (Fig. 5). The work notes an increase in \(\rho_v\) and a decrease in \(\operatorname{tg}\delta\) of this rubber after additional heating to a temperature of \(100^\circ\)C, which may be connected with the continuing process of vulcanization during heating of the rubber.
In the work, a comparison was made of the properties of ethylene-propylene rubber based on a terpolymer as a function of the unsaturation
Fig. 5. Change in tensile strength and relative elongation of rubbers during thermal aging at \(120^\circ\)C. For designations, see Fig. 1
(from 0.86 to 1.13 mol%) of the latter. A change in the unsaturation of the rubber from 0.86 to 1.13 mol% does not lead to a significant change in the dielectric and mechanical properties of EPTR rubber.
EPTR rubber based on ethylene-propylene rubber has fairly high dielectric properties, but low mechanical strength (Table 1).
In the work, a comparison was made of the properties of EPR rubber as a function of the hardness of the rubber. The dielectric properties of the rubber do not depend on the hardness of the rubber, while the mechanical properties improve with increasing rubber hardness: an increase in Defo hardness from 280 g to 800 g leads to a 44% increase in tensile strength, a 30% increase in relative elongation, and a twofold increase in residual elongation of EPR rubber. In addition, replacement in these rubbers of the filler—chemically precipitated chalk—with white carbon black of the Aerosil type leads to a significant increase in mecha-
mechanical strength (by 75%), relative elongation (by 50%), and residual elongation (by a factor of 3).
Thus, it is possible to obtain an ozone- and corona-resistant ethylene-propylene rubber with high dielectric and mechanical properties.
In this work, a comparison was made of the properties of two butyl-rubber rubbers with different vulcanizing groups: 1) sulfur-free, consisting of para-quinone dioxime, Altax, and manganese peroxide; 2) sulfur-containing, consisting of para-quinone dioxime and sulfur.
In Figs. 1–5 and in Table 1, data are given for BR rubber with a sulfur-free group.
The second butyl-rubber rubber has poorer electrical-insulating and mechanical properties than the first BR rubber (sulfur-free):
\(U_{\text{br}}\) is lower by 10%, \(\rho_v\) is lower by a factor of 4, \(\operatorname{tg}\delta\) at \(100^\circ\text{C}\) is higher by a factor of 4, \(\varepsilon\) is higher by a factor of 1.5; tensile strength, relative elongation, and residual elongation are lower by 10, 20, and 18%, respectively, and in the course of thermal aging at \(120^\circ\text{C}\) for 4 days they decrease by 28, 37, and 45%, respectively (for the sulfur-free rubber the decreases under thermal aging are 3.0, 7.0, and 5.8%).
The increased heat resistance of BR rubbers with a sulfur-free group, compared with sulfur-containing BR rubber, is explained by the cross-links \(-C-C-\) and \(C-O-C-O\), which are more resistant to thermochemical action, compared with the bonds \(-C-S-C-\). In addition, in rubbers with a sulfur-free vulcanizing group the residual unsaturation is lower than in sulfur-containing rubbers [1]. Therefore sulfur-free BR rubber is more ozone-resistant than sulfur-containing BR rubber.
In the work, a noticeable deterioration of the properties of sulfur-containing BR rubber was obtained when the thickness of the specimens was increased from 1 to 5 mm, which indicates an increase in the nonuniformity of the insulation with increasing thickness. This is also confirmed by the larger value of the root-mean-square deviation of \(U_{\text{br}}\) for the sulfur-containing rubber, amounting to 4.3 kV at \(U_{\text{br, av.}} = 35\) kV (for the sulfur-free rubber, 3.75 kV at \(U_{\text{br, av.}} = 38\) kV).
A study of the change in the mechanical properties of rubbers during thermal aging at \(120^\circ\text{C}\) showed (Fig. 5) that, with aging time, rubbers based on rubbers having unsaturation exhibit a decrease in tensile strength (TSP, BR, EPTR rubbers), while \(\sigma\) for rubbers based on saturated rubbers (EPR, KO-1, KO-2 rubbers) increases with aging time, which may be associated with degradation processes in the first case and with structuring in the second case. In the course of thermal aging, a significant decrease in the relative and residual elongations of TSP, BR, and EPTR rubbers was observed. The relative elongation of organosilicon rubbers decreases only slightly with aging time, while that of EPR rubber increases slightly.
On the basis of the experimental data obtained, the following conclusions may be drawn:
- In terms of electrical-insulating properties, the best rubbers among those considered are butyl-rubber, ethylene-propylene rubber based on a ternary copolymer, and organosilicon rubber based on Silasten-50 rubber. However, EPTR rubber has a low tensile strength of \(34.8 \, \frac{\text{kg}}{\text{cm}^2}\) and a sharp decrease under thermal aging in relative and residual elongations.
-
The silicone rubbers studied, based on SKT and fluorine-containing rubber, have low electrical-insulating and mechanical properties (rubber with SKT).
-
A change in the unsaturation of terpolymer ethylene-propylene rubber from 0.86 to 1.13 mol% does not have a significant effect on the dielectric and mechanical properties of the rubber.
-
Increasing the Defo hardness of ethylene-propylene rubber from 280 G to 800 G leads to a 44% increase in tensile strength, a 30% increase in relative elongation, and a twofold increase in the permanent elongation of the rubber. Replacing chemically precipitated chalk in ethylene-propylene rubber with Aerosil-type white carbon black leads to an increase of 75% in $\sigma$, 50% in $\varepsilon_z$, and a threefold increase in permanent elongation.
-
Butyl-rubber stock with a sulfur vulcanizing group has poorer dielectric and mechanical properties, lower heat resistance, and greater inhomogeneity in comparison with sulfur-free BK rubber.
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- Report on a visit by cable specialists to factories in Italy. ONTI, 1966.
- Davini G. Contemporary development of high-voltage cables insulated with ethylene-propylene rubber. “Mater. plast. ed elast.,” 1967, 33, No. 3.
- L. I. Kranikhfeld, T. M. Orlovich, L. Z. Khazen. Silicone Rubbers in Cable Engineering. VNIIEM, 1966.