STUDY OF THE LONG-TERM ELECTRIC STRENGTH OF INSULATING RUBBERS

M. N. TRESKINA, N. R. GONCHAR, I. I. SINITSYN

(Presented by the scientific seminar of the Department of Electrical Insulation and Cable Engineering)

In practice, rubber cable insulation is subjected simultaneously to the action of electric and thermal fields and mechanical loads. Therefore, it is of interest to study the lifetime of rubber under conditions close to operating conditions. In the absence of external corona, the development of breakdown is governed by processes occurring in the volume of the rubber. Rubber is multicomponent, and owing to structural and technological features it is characterized by porosity. It may be assumed that the main factors leading to breakdown of rubber specimens will be the action of partial discharges in microvolumes and the destructive action of ozone.

The most widely used rubbers TSSh-35A and TSShM-40, based on rubbers with high unsaturation, were selected as the objects of study. These rubbers differ in the percentage of rubber in the compound; in addition, in the rubber TSShM-40 a strong antiozonant—Santoflex UR—was used. Many properties and the behavior of rubbers during aging are affected by the type of spatial crosslinking [1—3]. Thus, rubbers in which thiuram is used as the vulcanizing agent have predominantly mono- and disulfide bonds, whereas sulfur rubbers have polysulfide bonds. If the former rubber is characterized by a high energy of crosslinks and higher heat resistance, then sulfur rubbers, owing to good relaxation properties, have higher mechanical characteristics but are less resistant to thermal and ozone aging [1—3]. To study the influence of the type of spatial crosslinking on long-term electric strength, model rubbers No. 144 (thiuram) and No. 145 (sulfur) were used in this work. The composition of the rubbers studied is given in Table 1.

The rubber specimens were prepared in the laboratory of TomNIKI. The vulcanization regime in the press was \(143^\circ\text{C}\) for 30 min. To study electric strength, flat rubber specimens \(0.5 \pm 0.05\) mm thick were used. A uniform electric field was created by plane–plane electrodes: a) the lower electrode was a grounded metal plate; b) the upper high-voltage electrode was a brass cylinder 25 mm in diameter with rounded edges, compression-molded with insulating rubber to eliminate external corona.

At specified values of the electric-field strength, static tensile deformation, and temperature, a set of lifetimes to breakdown was determined (20—35 points); the value adopted as the estimate was the time corresponding to the ...

corresponding to probabilities \(P = 0.5\) and \(0.7\) from the integral curves of the lifetime distribution.

Table 1

Composition of the rubbers studied

Data on the “lifetime curves” of cables and various insulating materials are known in the literature. In cable engineering it is customary to present “lifetime curves” in the coordinates \(E = f(T)\), while straightening the curves

\[ E = f\left(\frac{1}{\sqrt[n]{T}}\right) \]

makes it possible to determine the long-term strength \(E_{\infty}\). In work with polymer films, the authors [4] investigated the dependences \(\tau = BE^{-m}\) and \(\tau = Ae^{-BE}\). Analysis of the dependence \(\lg \tau = f(\lg E)\) showed that, in a certain interval of \(E\), the dependence is a straight line. The lifetime \(\tau\) increases sharply, tending to infinity when a field strength close to the ionization-inception strength is applied. At \(E < E_i\), partial discharges are absent and ionization aging of polymer films does not occur [4].

In investigating the “lifetime curves” of rubbers, the dependences \(\lg \tau = f(E)\) and \(\lg \tau = f(\lg E)\) do not differ appreciably. However, the field strengths at which the lifetime tends to “infinity” are clearly apparent. We adopted the dependence \(\lg \tau = f(\lg E)\) for discussion. In their character, the dependences obtained are analogous to the curves discussed in [4].

In the stretched state, scattering and the formation of free volumes (pores) are possible within the bulk of the rubber, as is a stressed state of bonds in the molecular system. At certain electric-field strengths, ionization arises in the pores, and as a result of its destructive action (electron and ion bombardment, local heating, the action of discharge products, especially ozone), the rubber fails with time and breakdown occurs.

As can be seen from Figs. 1–3, at certain electric-field strengths the lifetime of rubbers is large, which is possible and is determined by incipient ionization that does not cause substantial destruction \((E_i)\). In agreement with the stated proposition is the fact that, for all

for rubbers, an increase in the static tensile deformation leads to a decrease in \(E_u\) (Figs. 1–2). The deformations were chosen close to those practically encountered in the operation of flexible cables (10–20%). An increase in deformation from 10 to 20% leads to a decrease in \(E_u\) by 20% at a temperature of 20°C and by 8–10% at 60°C, and, at a given field strength,

Fig. 1. Dependence \(\lg \tau = F(\lg E)\) for rubber TSh-35A:
1 — \(t = 20^\circ\)C, \(\varepsilon = 10\%\); 2 — \(t = 20^\circ\)C, \(\varepsilon = 20\%\); 3 — \(t = 60^\circ\)C, \(\varepsilon = 10\%\); 4 — \(t = 60^\circ\)C, \(\varepsilon = 20\%\).

such an increase in deformation shortens the lifetime of the rubbers by a factor of 10 at 20°C and by a factor of 2.5 at 60°C. The insignificant changes at 60°C may be associated with relaxation processes, with a reduction of internal stresses in the stretched rubbers.

The stated proposition concerning the nature of failure is also confirmed by the fact that rubber with an antiozonant (TShM-40) is characterized by higher \(E_u\) and longer lifetimes (especially at a temperature of 20°C). With increasing temperature, the effectiveness of the protective action of Santoflex decreases, and the difference in the values of \(E_u\) for rubbers TShM-35A and TShM-40 becomes less significant (Figs. 1, 2).

Figure 3 presents the dependences \(\lg \tau = f(\lg E)\) for rubbers with different vulcanizing groups. Both at a temperature of 20°C and in the heated state (60°C), higher field strengths (\(E_u\)) and longer lifetimes are characteristic of thiuram rubber (No. 144) with mono- and disulfide cross-links, which have a high bond energy. Although the primary acts of destruction of vulcanizates are due to double bonds, in the process of bulk destruction

(for example, during the growth of ozone cracks leading to breakdown) the magnitude of the energy of spatial crosslinking of molecules contributes and, apparently, stress relaxation at the crack tip due to the relaxation capabilities of the chains has a lesser effect (for rubber No. 145 the lifetimes are shorter than for rubber No. 144).

Fig. 2. Dependence \(\lg \tau = f(\lg E)\) for rubber TSShM-40-2:
\(1 — t = 20^\circ\text{C}, \varepsilon = 10\%\); \(2 — t = 20^\circ\text{C}, \varepsilon = 20\%\); \(3 — t = 60^\circ\text{C}, \varepsilon = 10\%\); \(4 — t = 60^\circ\text{C}, \varepsilon = 20\%\)

Analyzing the values of the electric-field strengths \(E_u\) at which the lifetimes of the rubbers tend to “infinity” (more than 100 h), one may recommend a procedure for determining the permissible operating field strengths \(E_{\max}\).

Table 2 gives the values of \(E_u\) obtained in this work for the rubbers studied.

It is known that dynamic and cyclic deformation are more dangerous for rubbers than static deformation [3]. Unfortunately, there are no coefficients that would make it possible to convert the durability of rubbers when passing from one type of deformation to another. Taking into account the possible exposure of rubber in service not only to static deformations but also to cyclic and dynamic ones, as well as possible electrical overvoltages during cable operation, it is necessary, when determining \(E_{\max}\), to provide for a strength safety factor \(n\).

Fig. 3. Dependence \(\lg \tau = f(\lg E)\) at \(\varepsilon = 20\%\) for rubbers No. 144 \((1—20^\circ\mathrm{C},\ 2—60^\circ\mathrm{C})\) and No. 145 \((3—20^\circ\mathrm{C},\ 4—60^\circ\mathrm{C})\)

Table 2

Values of the ionization-inception field strengths, determined from the “life curves” of the rubbers

Knowing that for flexible cables rated at 6 kV with insulation made of rubber TSSH-35A, \(E_{\max} = 1.75 \div 2.5\) kV/mm is permissible, \(n\) can be determined from the experimental data (Table 2) when testing rubbers at \(t = 60^\circ\mathrm{C}\) and \(t = 20\%\).

\[ n = \frac{E_u}{E_{\max}} = \frac{5.6}{1.75 \div 2.5} = 3 \div 2, \]

Then for rubber TSHM-40 the following may be recommended:

\[ E_{\max} = \frac{8.7}{3 \div 2} = 2.9 \div 4 \text{ kV/mm}. \]

Conclusions

For rubbers, a substantial dependence of service life on the magnitude of the electric-field strength, static tensile deformation, temperature, the presence of the antioxidant Santoflex \(JP\), and the type of vulcanizing bridges has been observed:

a) an increase in deformation from 10 to 20% leads to a 100-fold decrease in \(E_u\) of TSHM-40 rubber containing Santoflex \(JP\); this effect is less substantial at 20%;

b) at identical field strengths, the service life of TSHM-40 rubber containing Santoflex \(JP\), stretched by 20%, is 100 times greater than the service life of TSSH-35A rubber at \(20^\circ\mathrm{C}\), and 16 times greater at \(60^\circ\mathrm{C}\);

c) at \(\varepsilon = 20\%\), \(t = 60^\circ\mathrm{C}\), the field strengths \(E_u\) corresponding to \(\tau \to \infty\) are 5.6 and 8.7 kV/mm for TSSH-35A and TSHM-40, respectively. If \(E_{\max}\) of TSSH-35A rubber is 1.75–2.5 kV/mm, then for TSHM-40, 2.9–4 kV/mm can be recommended;

d) rubber in which thiuram is the vulcanizing agent is more resistant to the action of an electric field than rubber vulcanized with sulfur.

References

1. A. Z. Borshchevskaya. Corrosion failure of stressed rubbers. Candidate’s dissertation, Moscow, NIIRP, 1965.

2. A. S. Kuzminskii, N. N. Lezhnev, Yu. S. Zuev. Oxidation of Rubbers and Rubber Products. Moscow, Goskhimizdat, 1957.

3. S. M. Kavun. Study of the thermal and thermo-oxidative destruction of cross-linked elastomers under conditions of static and dynamic loading. Candidate’s dissertation. Moscow, IKHFAN, 1968.

4. S. N. Koikov, A. N. Tsikin. Electrical Aging of Solid Dielectrics. Leningrad, “Energiya,” 1968.