Changes in the IR Spectra of Vulcanizates with the Time of Ionization Aging

N. R. Gonchar, G. F. Ivanov, M. N. Treskina, O. N. Sokolovskii

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

In the rubber insulation of cables, when an electric field is applied, ionization of gaseous inclusions is possible. In this case the rubber is subjected to electron and ion bombardment, local heating, and the action of discharge products, for example ozone. The literature contains no data on the nature of the structural-chemical changes occurring in rubber insulation under the action of the ionization process as a whole. It is known [1, 2] that the change in the structure of polymer films with the time of ionization aging is of decisive importance.

In the present work, changes in the IR spectra of unfilled vulcanizates of various compositions (Table 1) were studied as a function of the time of ionization aging. The rubber compound prepared on microrolls was dissolved in carbon tetrachloride and, by pouring the solution onto a NaCl substrate and gradually evaporating the solvent, a film 30–50 μm thick was obtained; it was then vulcanized in a thermostat according to the regime indicated in Table 1.

The vulcanizate film was subjected to ionization at room temperature and atmospheric pressure. The film with the substrate was placed—

Table 1

Composition of rubber compounds and vulcanization regime

were placed between flat electrodes; above the film a closed gap 1 mm thick was created. A voltage of industrial frequency was applied to the electrodes, equal to 1.5 times the ionization inception voltage and amounting to 9.2 kV rms.

IR spectra were recorded on an IKS-14A spectrophotometer in the NaCl and LiF prism range. To record the IR spectrum, the vulcanizate film was periodically removed from the test cell, and the same portion of the film was subjected to spectrophotometry. In the IR spectra of vulcanizate films subjected to ionization aging, substantial changes are observed in comparison with the spectra of the original specimens (Fig. 1). To evaluate the changes in the most characteristic groups, the optical density was determined at the corresponding wavelength of the vulcanizate film at different stages of aging. The results obtained in the work are presented in Fig. 2.

Fig. 1. Change in the IR spectrum of an SKEPT-based vulcanizate (specimen No. 6) with ionization-aging time. 1 — original film; 2 — after 4 h of ionization aging; 3 — after 12 h of ionization aging.

As can be seen from Figs. 1 and 2, during ionization aging an increase is observed in the absorption band at 3400 cm\(^{-1}\), characteristic of stretching vibrations of hydroxyl groups in alcohols [3]. As the intensity increases, the maximum of the band in specimens Nos. 1, 2, and 4 shifts into the short-wave region, to 3500 cm\(^{-1}\), which is characteristic of stretching vibrations of the hydroxyl group in acids [4–5]. At the same time, a significant increase occurs in the intensity of the absorption band at 1308 cm\(^{-1}\), corresponding to in-plane deformation vibrations

Figure 2. Kinetics of the accumulation of oxygen-containing and terminal methyl groups during ionization aging of vulcanizates. \((D_0\) and \(D_\tau\) are the optical densities before aging and at time \(\tau\).)

In the figure, the ordinate is \((D_\tau - D_0)/D_0\), and the abscissa is \(\tau\), h, up to 28 h.

Sample number / composition:

1. NR + SKA + thiuram
2. NR + SKD + sulfur
3. NR + SKA + thiuram + Santomex DR
4. NR + SKA + sulfur + Santomex DR
5. SKEPT + peroxide
6. SKEPT + sulfur

Functional group or bond:

• \(-\mathrm{OH}\) \((3400\ \mathrm{cm}^{-1})\)
• \(>\mathrm{C}=\mathrm{O}\) \((1709\ \mathrm{cm}^{-1})\)
• \(-\mathrm{CH}_3\) \((1375\ \mathrm{cm}^{-1}\) for NR + SKD; \(1380\ \mathrm{cm}^{-1}\) for SKEPT)
• \(>\mathrm{C}-\mathrm{O}-\mathrm{C}<\) \((1170\ \mathrm{cm}^{-1})\)

hydroxyl groups in alcohols [3], and the absorption band at 1407 cm\(^{-1}\), apparently due to deformation vibrations of hydroxyl groups in acids [3]. Absorption increases in the region 1715–1670 cm\(^{-1}\) for vulcanizates based on NR with SKD (samples 1–4) and in the region 1750–1700 cm\(^{-1}\) for vulcanizates based on SKEPT (samples 5–6), which is characteristic of stretching vibrations of C=O in the carbonyl and carboxyl groups of unsaturated and saturated compounds, respectively [3–4].

Along with an increase in the amount of oxygen-containing groups with aging time, an increase is observed in terminal methyl groups, whose vibrations correspond to the absorption band at 1375 cm\(^{-1}\) in the case of NR+SKD vulcanizates and at 1380 cm\(^{-1}\) in the case of SKEPT vulcanizates [3–5].

The formation of carbonyl and carboxyl compounds in vulcanizates during ionization aging, as well as the significant growth of terminal methyl groups, indicates the predominance of destructive processes leading to rupture of the molecular chains of the rubber. As can be seen from Fig. 2, destruction of the polymer chain during ionization aging is expressed to a greater extent in the vulcanizate based on NR+SKD+thiuram (sample No. 1). The introduction of the stabilizer santoflex IP into this vulcanizate helps reduce the growth in intensity of the absorption bands characteristic of vibrations of oxygen-containing and terminal methyl groups (sample No. 3), which indicates a slowing of destructive processes in this vulcanizate. At the same time, in the vulcanizate based on NR+SKD with sulfur, santoflex IP is ineffective (samples No. 2 and No. 4), which can be explained by the antagonistic action of polysulfides and amine-series stabilizers [6].

In the region 1300–1000 cm\(^{-1}\), with ionization aging time, an increase in overall absorption is observed, more pronounced in the case of samples based on NR+SKD and SKEPT vulcanized with sulfur (samples No. 2 and No. 6). The increase in overall absorption in this region is an indicator of the appearance of cross-linked structures. The absorption bands standing out against this background at 1130 and 1090 cm\(^{-1}\) for vulcanizates based on NR+SKD and at 1170 and 1105 cm\(^{-1}\) for vulcanizates based on SKEPT probably correspond to stretching vibrations of the oxygen bridge C-O-C in complex and simple ethers [5]. Consequently, under the action of ionization processes in an oxygen-containing medium, crosslinking of polymer chains through oxygen may occur.

Judging from the absorption bands at 965, 840, 3085, and 3025 cm\(^{-1}\) (deformation and stretching vibrations of C—H bonds in the structures R\(_1\)CH=CHR\(_2\) and R\(_1\)R\(_2\)C=CHR\(_3\)), the unsaturation of vulcanizates based on NR+SKD changes insignificantly during ionization aging. However, the intensity of the absorption band at 1650 cm\(^{-1}\), characteristic of stretching vibrations of the C-C double bond, increases noticeably, and at the same time its broadening into the region of lower frequencies is observed. It is known [4, 5] that the intensity of the band of stretching vibrations of the C-C bond strongly depends on the molecular environment (the presence of conjugated double bonds, displacement of the double bond to the end of the chain). Evidently, interaction of the polymer chain with oxygen during ionization aging lowers the symmetry of the molecule with respect to the double bond, as a result of which the same or a smaller number of double bonds gives an absorption band of greater intensity.

The experimental data obtained in this work show that, with ionization aging time, the structure of the vulcanizates

changes significantly: accumulation of oxygen-containing groups occurs (predominantly hydroxyl groups for NR+SKD and carbonyl groups for SKÉPT), destruction of the molecular chains of the rubber, and crosslinking of molecular chains through oxygen bridges. The nature and rate of accumulation of oxygen-containing groups and the ratio of the processes of destruction and structuring depend on the composition of the vulcanizates: the type of rubber and vulcanizing system, and the presence of a stabilizer.

REFERENCES

1. S. N. Koikov, A. N. Tsikin. Electrical Aging of Solid Dielectrics. Leningrad, Energiya Publishing House, 1968, pp. 123–130.

2. M. A. Bagirov, V. P. Malin, E. Ya. Volchenkov, Yu. N. Gazaryan. Investigation of the ionization aging of polystyrene film by IR spectroscopy. Vysokomolek. soed., A11, 1969, No. 10.

3. L. Bellamy. The Infrared Spectra of Complex Molecules. Moscow, Izd-vo IL, 1963.

4. L. Bellamy. New Data on the IR Spectra of Complex Molecules. Moscow, Mir Publishing House, 1971.

5. J. Brand, G. Eglinton. Applications of Spectroscopy in Organic Chemistry. Moscow, Mir Publishing House, 1967.

6. I. I. Cupeer. Rubb. Chem. Technol., 41, No. 1, p. 182, 1968.