EFFECT OF THE VULCANIZING SYSTEM ON THE PROPERTIES OF INSULATING RUBBERS BASED ON BUTYL RUBBER

N. R. GONCHAR, M. N. TRESKINA

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

The effect of the vulcanizing system on the dielectric, physicomechanical properties and ionization resistance of insulating rubbers based on butyl rubber with an unsaturation of 0.97 molar % was studied. The vulcanizing systems most widely used for cable insulation [1, 2], capable of causing the formation of chemical cross-links of different nature, were investigated. The composition of the rubbers is given in Table 1.

Table 1

Composition (in parts by weight) of rubbers based on butyl rubber

It is known [3, 4] that vulcanization of rubbers based on butyl rubber with sulfur (rubber BKR-3) leads to the formation of polysulfide cross-links of the type $C—S_x—C$, where $x = 2$ and more, whereas during vulcanization with parachinone dioxime (rubber BKR-2) predominantly $C—N$ bonds are formed. Rubber BKR-1 has a combined vul-

vulcanizing system and, apparently, during vulcanization crosslinks of both of the above-mentioned types may arise in it, i.e., C—S$_x$—C and C—N.

The mobility of segments of molecular chains, as well as the strength of the crosslink, depends on the structure of the vulcanization bridge. Consequently, one may expect the structure of the crosslinks to affect the properties of insulating rubbers. In studying the properties of insulating rubbers the following methods were used.

The specific volume resistivity of the rubbers was calculated from currents measured with a tube electrometer after holding the specimen under voltage for 1 minute. Measurement of $\operatorname{tg}\delta$ and $\varepsilon$ was carried out

Fig. 1. Dependence of the logarithm of specific volume resistivity $\lg \rho_v$ (—) and of the dielectric loss tangent $\operatorname{tg}\delta$ (---) of insulating rubbers based on butyl rubber on temperature. 1—BKR-1; 2—BKR-2; 3—BKR-3.

at industrial frequency using a high-voltage bridge of type R-525. The breakdown voltage of the specimens was determined on a high-voltage installation using plane–plane electrodes placed in dibutyl phthalate; the voltage was increased up to breakdown at a rate of 1 kV/sec. Measurements of the mechanical properties of the rubbers were carried out according to GOST 270—64. The ionization resistance of the rubbers was studied on flat specimens 1 ± 0.1 mm thick at industrial-frequency voltage using a needle–plane electrode system. The air gap between the surface of the rubber specimen and the high-voltage needle electrode was 1 mm. The ionization resistance of the rubber was evaluated by taking as the measure the lifetime of the specimens to breakdown under the action of open corona.

The values of $\rho_v$ (Fig. 1) at $20^\circ$C for rubbers BKR-1, BKR-2, and BKR-3 are close, and with increasing temperature they decrease similarly; however, the sulfur rubber BKR-3 has the smaller temperature dependence. Rubber BKR-1 has an elevated $\operatorname{tg}\delta$ and a considerable temperature dependence of $\operatorname{tg}\delta$ (Fig. 1). The dielectric permittivity ($\varepsilon$) of rubber BKR-1 increases from 3.1 to 3.6 as the temperature is raised from $+20$ to $+120^\circ$C. Consequently, with increasing temperature not only the conductivity increases, but also the polarization effects in this rubber.

Fig. 2. Integral distribution curves: a) tensile strength $\sigma_z$ (——) and relative elongation $\varepsilon_z$ (---); b) short-time electric strength $E_{\mathrm{pr}}$ for insulating rubbers based on butyl rubber. 1 — BKR-1; 2 — BKR-2; 3 — BKR-3.

The electric strength* ($E_{\mathrm{pr}}$ at $P = 0.5$) of the sulfur-free rubber BKR-2 is 1.4 times higher than that of the sulfur rubber BKR-3 (Fig. 2b). It is interesting to note here that the modulus at 300% elongation of rubber BKR-2 is approximately the same number of times greater than that of the sulfur rubber BKR-3 (33.2 and 20.7 kg/cm², respectively). Rubber BKR-1, with a combined vulcanizing system, occupies an intermediate position between BKR-2 and BKR-3 both in electric strength and in modulus (24.4 kg/cm²). Taking into account that the modulus of a vulcanizate is proportional to the number of crosslinks formed during vulcanization [4], it may be assumed that the electric strength of the vulcanizate depends on the density of the vulcanization network.

* For each rubber, statistical processing was performed on data from 50 specimens.

In terms of physicomechanical properties, sulfur rubber BKR-3 has clear advantages over BKR-1 and BKR-2 and is more elastic (Fig. 2a). The high tensile strength of sulfur vulcanizates is explained [3, 4] by the ability of the long flexible bonds of the type C—S\(_x\)—C that are formed to rearrange; in this connection, changes in the mutual arrangement of chains under mechanical loads can occur more readily in sulfur vulcanizates than in BKR-2 vulcanizates with less flexible and mobile bonds, which withstand lower stresses.

Table 2

Change in the tensile strength \((\sigma_z)\) of insulating rubbers based on butyl rubber with different vulcanizing systems as a function of temperature

A shortcoming of rubbers based on butyl rubber, irrespective of the vulcanizing system, is the considerable decrease in the physicomechanical indices of the rubbers with increasing temperature (Table 2).

The structure of the vulcanization bridge also explains the sharp difference in the behavior of butyl-rubber vulcanizates during aging. Owing to the higher thermal stability of the transverse C—N bonds, compared with sulfide bonds, BKR-2 vulcanizates are considerably superior in heat resistance to sulfur BKR-3 vulcanizates (Table 3). Rubber BKR-1 with a combined vulcanization system occupies an intermediate position in heat resistance between BKR-2 and BKR-3.

Table 3

Change in the retention coefficient of tensile strength \((K_1)\) of rubbers based on butyl rubber with different vulcanizing systems as a function of aging time in a thermostat at 120°C

In this work, distributions of the “lifetimes” of rubbers up to breakdown under the action of an electric field and open corona were obtained.

As a result of ionization of the air gap, the rubber is affected by electron and ion bombardment, local heating, and discharge products, for example ozone. Observations showed that, under prolonged exposure of the surface of rubbers subjected to static tensile deformation to ionization processes, oriented cracks, similar to ozone cracks, appear and with time lead to breakdown.

At low field strengths and low corona intensity, the rate of the destructive action of ionization decreases, which accounts for the longer “lifetimes” (Fig. 3b). With an increase in tensile strain from 10 to 20%, a significant decrease in lifetime is observed (the region of critical deformation); when the tensile strain is increased from 20 to 50%, \(\tau\) changes almost not at all (Fig. 3a).

Fig. 3. Dependence of \(\lg \tau\) on tensile strain \(\varepsilon\) (a) and voltage \(U\) (b) for insulating rubbers based on butyl rubber. 1 — BKR-1; 2 — BKR-2; 3 — BKR-3.

Sulfur rubber BKR-3 has lower ionization resistance, which is consistent with the reduced heat resistance and ozone resistance of this rubber in comparison with BKR-1 and BKR-2.

Thus, the studies showed that the structure of the vulcanization network affects the dielectric, physicomechanical properties and ionization resistance of insulating rubbers.

References

1. V. S. Penn. Technology for the Processing of Synthetic Rubbers. Moscow, Khimiya Publishing House, 1964.

2. P. M. Glupushkin, A. E. Saakyan, D. P. Shcherbakov. Cable Rubbers. Moscow, Energiya Publishing House, 1966.

3. Vulcanization of Elastomers. Edited by G. Alliger and I. Sbetun. Moscow, Khimiya Publishing House, 1967.

4. V. Hofmann. Vulcanization and Vulcanizing Agents. Leningrad, Khimiya Publishing House, 1968.