INFLUENCE OF INDIVIDUAL FACTORS OF THE IONIZATION PROCESS ON CHANGES IN THE STRUCTURE AND PROPERTIES OF INSULATING RUBBERS

M. N. TRESKINA, O. N. SOKOLOVSKII, N. V. EREMENKO, V. V. NOSOV

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

This paper presents the results of studies of changes in the structure and certain properties of vulcanizates under various aging conditions:

1) under the action of the full ionization process in a closed chamber in an air atmosphere;
2) under the action only of discharge products;
3) under the action of partial discharges in a nitrogen atmosphere;
4) under thermal aging at \(100^\circ\text{C}\).

For the study, a rubber model TShM-40 was selected, unfilled (No. 140) and filled (No. 144).

The composition of the vulcanizates studied is given in Table 1.

Table 1

Composition of TShM-40 rubber models

Weight, composition per 100 parts by weight of rubber.

The specimens were prepared by vulcanizing the rubber compound in a press. Taking into account that reactions during the ionization aging of rubbers will proceed from the surface layer, specimens of small thicknesses \((0.25 \div 0.35\ \text{mm})\) were selected for the studies, which increases the sensitivity of the characteristics being studied to changes in the structure of the vulcanizates.

It is known that for rubbers made on the basis of unsaturated rubbers, the reactions of interaction with ozone and oxygen are destructive. It may be assumed that during ionization aging the chemical mechanism of the action of partial discharges predominates. In this case one may expect the accumulation of oxygen-containing groups; processes of destruction of the linear chains of the polymer, destruction of sulfur bridges, and also crosslinking through oxygen bridges will occur. With the chemical nature of the action, the thermal factor and ultraviolet radiation may be regarded as initiating factors.

It is known that, when rubbers are irradiated with high-energy particles, radiation crosslinking is possible, accompanied by oxidative processes. In the infrared spectra of films of vulcanizates subjected to ionization aging, substantial changes are observed in comparison with the spectra of the original samples. Figure 1 presents—

Fig. 1. Kinetics of the change in optical density of individual absorption bands of vulcanizate 140 during ionization aging

—the kinetics of the change in optical density of individual absorption bands of vulcanizate No. 140 during ionization aging. An increase is observed in the absorption band at \(3400\ \text{cm}^{-1}\), characteristic of stretching vibrations of hydroxyl groups in alcohols [1]. At the same time, there is a significant increase in the intensity of the absorption band at \(1308\ \text{cm}^{-1}\), corresponding to in-plane deformation vibrations of hydroxyl groups in alcohols and to stretching vibrations of

\[ \begin{matrix} \\[-1.2em] \diagdown\!\!\diagdown \\ > \mathrm{C}-\mathrm{O} \end{matrix} \]

groups in acids [1]. Growth and broadening of the absorption band at \(1650\ \text{cm}^{-1}\) are observed, which indicates an increase in the number of \(\mathrm{C}=\mathrm{C}\) and \(\mathrm{C}=\mathrm{O}\) bonds conjugated with a double bond. Along with the increase in the number of oxygen-containing groups during aging, the formation and significant growth of terminal methyl groups are observed; their vibrations correspond to the absorption band at \(1375\ \text{cm}^{-1}\) and, apparently, at \(1362\ \text{cm}^{-1}\) [1]. The appearance of oxygen-containing

groups and degradation leads to an increase in the polarity of the system, as well as to dielectric relaxation and electrical conductivity. Rearrangement of the spatial crosslinking of the vulcanizate should change the activation energy both of dipole relaxation and of the electrical-conductivity process.

In this work, the change in electrical conductivity and dielectric losses of vulcanizates under the action of various factors of ionization aging was studied. The change in $\rho_v$ of unfilled and filled vulcanizates under all aging variants is shown in Figs. 2 and 3.

Fig. 2. Change in $\rho_v$ of vulcanizate 140 with aging time under the action of partial discharges (1), discharge products (2), partial discharges in a nitrogen atmosphere (3), and thermal aging (4).

As can be seen from Fig. 2, the change in $\rho_v$ of the unfilled vulcanizate under the action of complete ionization and of only discharge products is similar: a decrease in $\rho_v$ is observed both at 20°C and at elevated temperatures. During thermal aging at 100°C (in an air atmosphere), owing to post-vulcanization, $\rho_v$ increases; consequently, the thermal effect during the action of partial discharges on unfilled vulcanizates is of secondary importance.

Under the action of complete ionization or discharge products, oxidative processes lead to degradation of the linear polymer chain, as a result of which the activation energy of the conduction process decreases.

Oxidation of impurities leads to an increase in impurity conductivity (Fig. 2). The decomposition of hydroperoxides, isoozonide, and sulfur bonds leads to the formation of free radicals and low-molecular-weight polymer “fragments,” which is the reason for the increase in the high-temperature

Fig. 3. Change in \(\rho_v\) of vulcanizate 144 with aging time under the action of partial discharges (1), discharge products (2), partial discharges in a nitrogen atmosphere (3), and thermal aging (4)

electrical conductivity of the unfilled vulcanizate with aging time. In the process of exposure of vulcanizates to partial discharges in a nitrogen atmosphere, oxidative reactions are inevitable; therefore, the character of the change in \(\rho_v\) during prolonged aging is similar to aging under complete ionization.

When only the discharge products act on the filled vulcanizate (Fig. 3), \(\rho_v\) changes insignificantly with aging time, similarly to the effect of thermal aging at \(100^\circ\)C. Under the action of the complete ionization process, the changes in \(\rho_v\) are large. If aging of the filled vulcanizate in the discharge products leads at the first stage to a decrease in \(\rho_v\), then the influence of the total ionization effect leads to an increase in \(\rho_v\) at \(70^\circ\)C (Fig. 3). The character of the change in \(\rho_v\) during prolonged aging is also opposite. Consequently, the purely chemical nature of the interaction cannot be considered decisive in the ionization aging of filled vulcanizates. Thus, analyzing the character of the change in electrical conductivity during ionization aging, it may be considered that, if for unfilled vulcanizates the nature of aging has a cha

character (oxidation—destruction), then for filled ones the influence of electron and ion bombardment is substantial.

Figure 4 shows changes in $\operatorname{tg}\delta_{\max}$ and in the conductivity losses of vulcanizates with aging time under the action of various factors of the ionization process. As can be seen, in all cases an increase in $\operatorname{tg}\delta_{\max}$ and in conductivity losses is observed, the greatest being under the action of partial discharges in an air atmosphere and the smallest under the action of discharge products.

Fig. 4. Change in $\operatorname{tg}\delta_{\max}$ and $\operatorname{tg}\delta$ at $70^\circ\mathrm{C}$ of vulcanizates with aging time under the action of partial discharges (1), discharge products (2), and partial discharges in a nitrogen atmosphere (3).

discharge products. The data obtained can be explained as follows. The primary cause of the change in the structure of vulcanizates as a result of the action of partial discharges is electron–ion bombardment. However, for unprotected vulcanizate No. 140 the process of structural change is chemical in character; that is, the principal destructive agents are $\mathrm{O_2}$, $\mathrm{O_3}$, nitrogen oxides, etc. This is confirmed by the nature of the dependences $\operatorname{tg}\delta_{\max}=f(\tau)$ and $\operatorname{tg}\delta_{70^\circ\mathrm{C}}=f(\tau)$ for vulcanizate No. 140, which is similar for all three aging cases.

In protected vulcanizate No. 144, the dependences of the maximum dielectric losses and conduction losses are complex in character. Under the action of partial discharges in an atmosphere of an inert gas (nitrogen), the value of $\operatorname{tg}\delta_{\max}$ does not change with aging time. Under the action of other aging factors, however, it even decreases (which indicates crosslinking) in the first hours of aging, and then increases. Similar data were obtained in the aging of polymer dielectrics by partial discharges in [2]. It may be assumed that, during ionization aging of vulcanizate No. 144, the principal role in structural change is played by electron-ion bombardment, and only with more prolonged aging does the chemical nature of the aging mechanism begin to exert an increasing influence.

Comparing the changes in the properties of the unfilled vulcanizate under the action of the total ionization process and under the action only of discharge products (the changes in properties are similar), one may advance the supposition that the mechanism of destruction of the unfilled vulcanizate by partial discharges is chemical in nature. When partial discharges act on filled protected vulcanizates, the role of electron and ion bombardment is significant.

REFERENCES

1. L. Bellamy. Infrared Spectra of Complex Molecules. Moscow, IL, 1963.
2. V. M. Kirilenko. Investigation of Partial Discharges and Their Destructive Action on Polymer Dielectrics. Candidate’s dissertation. Kiev, 1969.