PROCEEDINGS
OF THE TOMSK POLYTECHNIC INSTITUTE
OF THE ORDER OF THE OCTOBER REVOLUTION AND THE ORDER
OF THE RED BANNER OF LABOR
NAMED AFTER S. M. KIROV

Volume 267 1975

STUDY OF CERTAIN ELECTROPHYSICAL PROPERTIES OF HOT-CURING EPOXY COMPOUNDS

R. M. KESSENIKH, L. A. KORSHUNOVA, A. V. PETROV

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

At the present time epoxy compounds have found wide application in electrical engineering as insulating materials. To reduce brittleness, plasticizers and modifiers are introduced into epoxy compounds; as is known, these alter the electrophysical properties of polymers, greatly lower their glass-transition temperature \(T\), and also exert a substantial influence on the mobility of the supramolecular structures of polymers [1, 2]. In view of the fact that the influence of plasticization and modification on the electrophysical properties of thermosetting epoxy polymers has been insufficiently studied [3–5], and the character of the supramolecular structures of plasticized and modified epoxy polymers has not been studied at all, it was of interest to investigate the effect of plasticization and modification on the electrical conductivity of epoxy polymers, their glass-transition temperature \(T_c\), and also on the nature of the change in the supramolecular structures of epoxy polymers. In addition, it was of interest to evaluate the mechanism of plasticization of an epoxy polymer and to find a correlation between the electrophysical properties of epoxy polymers and the character of the change in their supramolecular structures.

Experimental procedure

The investigations were carried out on an epoxy polymer based on the dian epoxy resin ED-6, cured with methyltetrahydrophthalic anhydride (MTGFA), which was introduced in an amount of 62 parts by weight per 100 parts by weight of ED-6. The curing of the specimens was carried out according to a stepwise regime at a temperature of \(120^\circ\mathrm{C}\) for 2 hours and \(150^\circ\mathrm{C}\) for 4 hours. Dibutyl phthalate (DBP), polyepoxy acrylate MGF-9, and the aliphatic epoxy resin DEG-1 were used as plasticizers and modifiers. The glass-transition temperature of the epoxy polymers was determined by the dielectric method. The residual current was measured by the procedure described previously [6].

The supramolecular structures were studied using a UEMV-100A electron microscope. Carbon replicas from the fracture surface, shadowed with platinum, were subjected to electron-microscopic examination. Fracture was carried out after cooling the specimens in liquid nitrogen.

Infrared spectra were recorded on an IKS-14 spectrophotometer in the range 600–4000 cm\(^{-1}\) (the finely ground powder was pressed in KBr).

Discussion of Results

Figure 1 shows the dependences of \(\lg \rho_v\) on the electric-field strength \(E\) for unplasticized (solid lines) and plasticized epoxy polymer (dashed lines). The dependences of \(\lg \rho\) on \(E\) obey Poole’s law. With the introduction of plasticizers and modifiers, the value of \(\rho_v\) decreases and the slope of the straight lines \(\lg \rho=f(E)\) changes. For all the polymers studied, the Poole coefficients were calculated by the least-squares method. The Poole coefficients depend on the type and amount of the plasticizers and modifiers introduced; moreover, with increasing plasticizer content, the maxima observed in the temperature dependences of the Poole coefficient shift toward lower temperatures.

Fig. 1. Dependence of \(\lg \rho_v\) on electric-field strength for pure epoxy polymer (solid lines) and polymer plasticized with 20 parts by weight of DBP (dashed lines).

The specific volume resistivity \(\rho_v\) at temperatures up to 70–80°C remains at a fairly high level (of the order of \(10^{13}\ \Omega\cdot\text{cm}\)) both for the plasticized and for the modified (even 40 parts by weight) epoxy polymer. At temperatures above 80°C a sharp decrease in \(\rho_v\) of the epoxy polymer is observed, especially for that plasticized with dibutyl phthalate. Thus, for example, when 40 parts by weight of DBP are introduced, \(\rho_v\) of the polymer at 100°C decreases to \(1\cdot 10^{11}\ \Omega\cdot\text{cm}\), whereas \(\rho\) of the unplasticized polymer remains 2.5 orders of magnitude higher. In the polymer modified with MGF-9, \(\rho_v\) is an order of magnitude higher than in the polymer plasticized with DBP.

The character of the change in the glass-transition temperature \(T_c\) with the content of plasticizers and modifiers is presented in Fig. 2. Introduction into the epoxy polymer of DBP up to 30 parts by weight and of the polyester MGF-9 up to 20 parts by weight strongly lowers the glass-transition temperature of the polymer; with a further increase in the content of DBP and MGF-9, \(T_c\) decreases only slightly.

The glass-transition temperature of the epoxy polymer modified with DEG-1 decreases monotonically with increasing content of the aliphatic resin.

It is currently believed that DBP reacts neither with the resin nor with the hardener, while the polyester MGF-9 interacts to a limited extent with the hardener. During the joint curing of epoxy dianic and aliphatic resins, crosslinked polymers with alternating aromatic and aliphatic units are formed. In order to assess

Fig. 2. Dependence of the glass-transition temperature \(T_g\) of an epoxy polymer on the content of plasticizers and modifiers: 1—DEG-1; 2—MGF-9; 3—DBP.

Fig. 3. IR absorption spectra of “pure” epoxy polymer [1], modified with 40 parts by weight of DEG-1 [2], and aliphatic epoxy resin DEG-1 [3].

structural changes occurring during the polymerization of the epoxy resin in the presence of plasticizers and modifiers, IR spectra were taken of the unplasticized and plasticized epoxy polymer, as well as spectra of the plasticizers and modifiers. The introduction of DBPT into the epoxy polymer did not change its spectrum at all, since all the main absorption bands of DBPT and of the resin cured with MTGFA coincide. In the spectrum of the polymer modified with the polyester MGF-9, small new bands appear at 1148 and 1087 cm\(^{-1}\), due to vibrations of ester groups; the band at 880 cm\(^{-1}\) is due to vibrations of the \(-\mathrm{C{-}CH_2}\) group, and the band at 753 cm\(^{-1}\) increases due to vibrations of ethylene groups. Using IR spectra, it was not possible to detect interaction of DBPT and MGF-9 with the anhydride. The spectrum of the epoxy polymer modified with the aliphatic epoxy resin DEG-1 (Fig. 3) does not contain the absorption bands at 917 and 866 cm\(^{-1}\), corresponding to vibrations of epoxy groups, and the absorption band at 1730 cm\(^{-1}\), corresponding to vibrations of carbonyl groups, increased sharply, which indicates interaction of the aliphatic resin with the anhydride. In addition, the absorption intensity increased for the bands at 1270 and 1316 cm\(^{-1}\), corresponding to vibrations of hydroxyl groups, and for the bands at 1052 and 1190 cm\(^{-1}\), corresponding to vibrations of ethylene groups.

Plasticization affects the character of the supramolecular structures. In the unplasticized epoxy polymer, a clearly expressed globular supramolecular structure is observed. The sizes of the globules are rather large, on the order of 300–1000 Å; the entire substance is in globular form, and the globules are located close to one another.

The introduction of DBPT strongly changes the morphology of the supramolecular structure. Aggregation of globules occurs into larger formations, reaching on the order of 5000–10000 Å.

Conclusions

Thus, the study of the influence of plasticizers and modifiers on the properties of a hot-cured epoxy polymer makes it possible to conclude that there is a correlation between the properties and the structure of the plasticized and modified systems studied; moreover, the regularities observed in the plasticization of thermoplastic polymers also occur for thermosetting polymers [7–9].

REFERENCES

1. V. A. Kargin, G. M. Slonimsky. Brief Essays on the Physical Chemistry of Polymers. “Khimiya” Publishing House, 1967.

2. A. A. Tager. Physical Chemistry of Polymers. “Khimiya” Publishing House, 1968.

3. I. M. Gurman, T. S. Khramova, M. S. Akutin et al. “Plasticheskie massy,” No. 5, 24, 1968.

4. S. N. Antonov, I. M. Gurman, V. V. Kovriga. “Plasticheskie massy,” No. 2, 37, 1967.

5. R. M. Kessenikh, L. A. Korshunova, A. F. Kalganov. “Plasticheskie massy,” No. 10, 46, 1969.

6. R. M. Kessenikh, L. A. Korshunova, A. F. Kalganov. Izv. vuzov, “Fizika,” No. 11, 129, 1967.

7. R. M. Asimova, P. V. Kozlov, V. A. Kargin, S. M. Vtorygin. High-Molecular Compounds. “Nauka” Publishing House, 4, 554, 1962.

8. I. N. Razinskaya, P. V. Kozlov, B. P. Shtarkman, L. P. Ignat’eva. High-Molecular Compounds. 5, 1850, “Nauka” Publishing House, 1963.

9. A. A. Askadsky. Physical Chemistry of Polyarylates. “Khimiya” Publishing House, 1968.