SOME ISSUES IN THE STUDY OF ANTIOZONANTS IN INSULATING RUBBERS

N. R. GONCHAR, K. M. KEVROLEVA, M. N. TRESKINA

(Presented by the Joint Scientific Seminar of the Department of Electrical-Insulation and Cable Engineering)

One of the effective stabilization methods that protect rubber products from ozone cracking is the inhibition of the chemical interaction of ozone with polymers by introducing specific chemical compounds—antiozonants—into rubber compounds.

The hypothetical mechanisms of the protective action of antiozonants that are available in the literature, and that are considered the most probable, still do not fully and precisely elucidate the true nature of the protective action [1]. Some authors [2] believe that, under the action of ozone, the chain of the rubber molecule is broken or cleaved, while antiozonants promote restoration of the chain—in their absence, cracks are formed. In addition, there is the view that antiozonants provide additional protection by reacting with ozone and forming a protective film on the surface of the rubber. This is explained by the fact that ozone reacts more rapidly with the antiozonant than with the polymer. Therefore, the protective properties of an antiozonant are the greater, the higher its reactivity toward ozone and the more molecules of it migrate to the rubber surface [3].

According to other authors [4], the mechanism of action of antiozonants apparently cannot be reduced to simple ozone capture. Confirmation of this is at least the fact that the reaction product of acetone with diphenylamine is not the best antiozonant, although vulcanizates containing it (Table 1) absorb significantly more ozone than vulcanizates containing generally recognized antiozonants.

The results of the study show that no correlation exists between ozone absorption and ozone resistance.

The fact that the amount of ozone absorbed by undeformed specimens depends little on the presence of antiozonants, while under 6% tensile deformation it is noticeably greater for specimens containing antiozonants, suggests that the action of antiozonants also apparently cannot be reduced to the formation of a protective film as a result of the reaction of ozone with the antiozonant. Possibly, antiozonants react with the zwitterion formed in the decomposition reaction of an olefin according to the Criegee mechanism [4]:

\[ >C=C< + O_3 \to C=O + >C< \begin{matrix} O - O^{\ominus}\\ O^{\oplus} \end{matrix} \]

The possibility of such a reaction was shown in [5].

The effect achieved when antiozonants are introduced into rubbers depends on their structure. Thus, derivatives of \(N, N^{1}\)-\(n\)-phenylenediamine shift the region of maximum cracking toward larger deformations, while at the same time decreasing the absolute value of the cracking maximum.

Table 1

Results of a study of ozone absorption and cut growth in natural-rubber rubbers containing various protective substances

cracking. Antiozonants of the 6-alkoxy-2,2,4-trimethyldihydroquinoline group do not affect the position of the region of maximum cracking, but at all elongations they reduce the degree of cracking in comparison with the unprotected sample.

As is known [6], the most effective antiozonants currently used are derivatives of \(N, N^{1}\)-\(n\)-phenylenediamine and 6-alkoxy-2,2,4-trimethyldihydroquinolines.

In [7], the protective ability of antiozonants of these two groups was studied under static, dynamic, and alternating static-dynamic deformations. Interesting data were obtained, which are given in Table 2.

Studies carried out at TomNIIKGP established a definite regularity in the activity of antiozonants with respect to insulating rubbers based on various rubbers.

From Fig. 1*) it is evident that \(N\)-isobutyl-\(n\)-aminophenol in rubber based on NR + SKD exhibits the greatest activity, and the introduction of as little as 3 parts by weight of it is already sufficiently effective. In rubber based on NR + SKMS-10RPD, this antiozonant shows its effectiveness at an amount of 5 parts by weight. The activity of \(N\)-isobutyl-\(n\)-aminophenol is least of all manifested in rubber based on NR + SKB-rD (commercial rubber TShch-35A). Similar results were also obtained with other antiozonants.

On the other hand, in one and the same rubber the effectiveness of different antiozonants is manifested differently. The dependence shown in Fig. 2 of the ozone resistance of insulating rubber TShM-40 based on NR + SKMS-10RPD on the amount of various antiozonants shows that the most effective antiozonants proved to be \(N\)-isopropyl-\(N^{1}\)-phenyl-\(n\)-phenylenediamine (Santoflex IP) and \(N\)-H-octyl-\(n\)-aminophenol, whereas \(N, N^{1}\)-diamyl-\(n\)-phenylenediamine and \(N\)-isobutyl-

* In all figures, the horizontal axis gives parts by weight of antiozonants per 100 parts by weight of rubber.

Fig. 1. Dependence of the ozone resistance of rubbers based on various elastomers on the dosage of N-isobutyl-p-aminophenol.

1 — NR + SKD in a ratio of 1 : 1; 2 — NR + SKMS-10RPD in a ratio of 1 : 1; 3 — NR + SKB-rD (serial rubber TSP-35A)

Fig. 2. Dependence of the ozone resistance of rubber based on NR + SKMS-10RPD on the dosage of various antiozonants.

1 — N-isopropyl-N′-phenyl-p-phenylenediamine (Santoflex IP); 2 — N-octyl-p-aminophenol; 3 — N, N′-diamyl-p-phenylenediamine; 4 — N-isobutyl-p-aminophenol.

Fig. 3. Dependence of the physical-mechanical characteristics of rubber on the dosage of \(N\)-isopropyl-\(N'\)-phenyl-\(p\)-phenylenediamine:
○—○ — tensile strength, \(\sigma\); ×—× — relative elongation, \(\varepsilon\); △—△ — residual elongation, \(Q\);
—— before aging; – – – after aging in a thermostat for 4 days at a temperature of \(120^\circ\mathrm{C}\).

Fig. 4. Dependence of the physical-mechanical characteristics of rubber on the dosage of 6-ethoxy-2,2,4-trimethyl-1,2-dihydroquinoline:
○—○ tensile strength, \(\sigma\); ×—× relative elongation, \(\varepsilon\); △—△ residual elongation, \(Q\);
—— before aging; – – – after aging in a thermostat for 4 days at a temperature of \(120^\circ\mathrm{C}\).

![Figure 5 graph]

Fig. 5. Dependence of the physicomechanical characteristics of rubber on the dosage of N-n-octyl-p-aminophenol:
○—○ tensile strength, $\delta$; ×—× relative elongation, $\varepsilon$; △—△ residual elongation, $Q$; —— before aging; — — — after aging in a thermostat for 4 days at a temperature of 120°C

![Figure 6 graph]

Fig. 6. Dependence of the dielectric characteristics and ozone resistance of rubber on the dosage of N-isopropyl-N'$-phenyl-p*-phenylenediamine: ○—○ $\rho_v$, ohm·cm; ×—× $\operatorname{tg}\delta$; △—△ ozone resistance at an ozone concentration of 0.015% by volume, minutes

Ozone, \(\tan\delta \cdot 10^3\), \(\rho_v \cdot 10^{14}\ \Omega\cdot\text{cm}\)

Weight parts

Fig. 7. Dependence of the dielectric characteristics and ozone resistance of rubber on the dosage of 6-ethoxy-2,2,4-trimethyl-1,2-dihydroquinoline;
○—○ \(\rho_v\), \(\Omega\cdot\text{cm}\); ×—× \(\tan\delta\); Δ—Δ ozone resistance at an ozone concentration of 0.015% by volume, minutes

Ozone, min.; \(\tan\delta \cdot 10^3\), \(\rho_v \cdot 10^{14}\ \Omega\cdot\text{cm}\)

Weight parts

Fig. 8. Dependence of the dielectric characteristics and ozone resistance of rubber on the dosage of \(N\)-\(n\)-octyl-\(p\)-aminophenol;
○—○ \(\rho_v\), \(\Omega\cdot\text{cm}\); ×—× \(\tan\delta\); Δ—Δ ozone resistance at an ozone concentration of 0.015% by volume, minutes

$n$-aminophenol must be introduced into the rubber compound in a large amount.

Table 2

Activity of protective compounds as antiozonants in butadiene–styrene rubber under different types of deformation

The results of a study of the influence of antiozonants of various groups on the physicomechanical and dielectric properties of insulating rubber TSShM-40 based on NR + SKMS-10RPD are given in Figs. 3–8.

Analysis of the data shows that, with an increase in the amount of antiozonants in rubber compounds, a certain decrease in the strength of the vulcanizates and an increase in relative elongation are observed; however, these changes are insignificant.

A general regularity to which attention must be paid is the deterioration of the dielectric properties of the rubber with an increase in the amount of antiozonant. It was noted that 6-ethoxy-2,2,4-trimethyl-1,2-dihydroquinoline (Santoflex AW) reduces the volume resistivity to a greater extent than $N$-isopropyl-$N^1$-phenyl-$n$-phenylenediamine (Santoflex JP) and $N$-H-octyl-$n$-aminophenol.

Thus, the use of antiozonants is promising, since it makes it possible to obtain ozone-resistant insulating rubbers based on non-ozone-resistant dielectric rubbers.

REFERENCES

1. M. A. Otopkova et al., Synthesis and Study of the Effectiveness of Stabilizers for Polymeric Materials. Voronezh, 1964.
2. Chemical Engineering, 69, No. 23, 1962, p. 86.
3. Chemistry and Technology of Polymers, No. 11, 1960.
4. Mc. Cool John C., Rubb. Chem. Technol., 37, No. 3, pp. 583–605, 1964.
5. Lorenz O., Park C. R., Rubb. Chem Technol., 36, p. 194, 1963.
6. Dibbo A., Gummi–Asbest–Kunststoffe, 18, No. 2, 1965.
7. Dibbo A., Transaction and Proceeding IRI, 40/11, No. 5, 1964.