STUDY OF THE ELECTRICAL AND MECHANICAL PROPERTIES OF A COMPOUND BASED ON CYCLOALIPHATIC EPOXY RESIN

Yu. N. Shumilov

(Presented by the scientific seminar of the Department of EIKT)

In recent years, both abroad and in the Soviet Union, a new class of cycloaliphatic epoxy resins has been created that do not contain phenolic rings. These resins possess high resistance to thermal degradation, are only weakly affected by ultraviolet irradiation, and make it possible to create compounds with high tracking resistance.

The study of the electrophysical characteristics of compounds based on cycloaliphatic resins is of great interest from both the scientific and the technical point of view. The present work gives the results of a study of the electrical and mechanical strength of unfilled compounds based on cycloaliphatic resin CY—180, manufactured by Ciba, Switzerland.

Resin CY—180 is a mixture of two components:

1. Araldite CY—175 resin (30%), having the structural formula:

1. A resin of the following chemical composition (70%).

The epoxy number of resin CY—180 was 21.8, and the epoxy equivalent was 192.5.

As hardeners, hexahydrophthalic anhydride (HHPA), chlorendic anhydride (HET), and their mixtures, taken in various percentage ratios, were used.

Hexahydrophthalic anhydride (HHPA).

Molecular weight — 154.
Melting point — 35°C.

Chlorendic anhydride (HET)

Molecular weight — 371.
Melting point — 238°C.

Chlorendic anhydride was chosen as the hardener for the following reasons. In our assumption, a certain amount of chlorine introduced into the molecular chain of the compound together with the hardener may lead to an increase in the forces of intermolecular interaction through the formation of strong bonds (nodes) between neighboring units and molecular chains of the polymer. This may cause an increase in the strength limit under brittle fracture and an increase in relative elongation under forced-elastic deformation of the polymer [1, 2]. In addition, the electronegative chlorine atoms fixed in the structure of the compound may lead to an increase in the concentration of traps for conduction electrons that appear in the dielectric at electric-field strengths corresponding to the region in which the Poole and Frenkel law is obeyed [3, 4]. An increase in the concentration of traps may raise the resistance of the compound to the formation of tree-like shoots—dendrites—which are the initial stages of destruction of epoxy insulation under prolonged exposure to alternating voltage.

The use of chlorendic anhydride is also attractive from the point of view of the possibility of increasing heat resistance and improving the volume resistivity of the epoxy compound [5, 6, 7]. In addition, the eutectic melt of chlorendic anhydride with hexahydrophthalic anhydride is liquid at room temperature, which facilitates work with the indicated mixture of hardeners.

Table 1 gives the formulations of the compounds studied and the conditions for their curing.

In preparing the specimens, the mixture was not vacuum-treated. After curing for 24 h, the specimens were removed from the casting molds and post-cured for 76 h at a temperature of 100°C.

For comparison of the properties of the compounds studied, it was desirable to have specimens with a similar degree of curing. Therefore, as the first stage of the work, we carried out studies of the curing kinetics of the compounds in order to select polymerization conditions providing a similar degree of spatial crosslinking. The kinetics and degree of curing of the specimens were assessed by determining, during curing, the infrared spectra in the range of wave numbers \(910\ \mathrm{cm}^{-1}\), which make it possible to trace the disappearance of epoxy groups during curing of the epoxy resin. It was established,

Table 1

Formulation and curing conditions of the compounds studied

Note: the content of chlorendic anhydride in the mixture is given in parentheses as a percentage of the calculated stoichiometric ratio.

that the visible change in the number of epoxy groups under this curing regime ceases after the following times: 100 h for formulation 1; 100 h for formulation 2; 20 h for formulation 3; and 2 h for formulation 4. Assuming that after the indicated times further curing of the compounds proceeds very slowly, we chose a curing time of 100 h for all formulations, and considered that in this case the temperature regime given in Table 2 made it possible to obtain specimens with the most nearly similar degree of cure.

It is useful to note that in this case chlorendic anhydride played the role of an accelerator of the curing reaction.

For studies of electrical strength, two types of electrodes were used: point—plane and sphere—plane. For the point—plane electrodes the radius of curvature of the point was within 15–78 microns and the distance between the electrodes was 0.1–0.8 mm. For the sphere—plane electrodes the radius of the sphere was 7.5 mm and the distance between the electrodes was 0.3 mm.

Measurement of electrical strength was carried out in transformer oil at room temperature. A voltage with a frequency of 50 Hz was applied smoothly up to breakdown at a rate of 2 kV/min. A layer of Aquadag was used as the electrodes.

Figure 1a presents the results of measuring the electrical strength of the compound as a function of the parts by weight of chlorendic anhydride in the eutectic hardener blend. It is clear from the figure that the average value of the electrical strength of the compounds increases with increasing percentage content of chlorendic anhydride in the mixture. The maximum value of the average electrical strength occurs for the compound containing 38.4 parts by weight (wt. parts) of anhydride (20% of the stoichiometric ratio), and for point—plane electrodes the electrical strength increases by a factor of 2.5; for electro-

...to the sphere—plane electrode system—by approximately a factor of 1.7. Thus, the addition of 20% chlorendic anhydride to the compound formulation leads to a noticeable increase in electrical strength both in a sharply nonuniform and in a weakly nonuniform electric field.

In parallel, for these same compositions, the mechanical characteristics at room temperature were determined: tensile strength \(\sigma_p\), flexural strength \(\sigma_u\), and relative elongation \(\varepsilon\). In determining the indicated characteristics, a “load–deformation” diagram was recorded for each specimen.

Figure 1,a gives the mean values of the indicated characteristics corresponding to measurements performed on 5–7 specimens. From Fig. 1,a it is evident that, with an increase in the chlorendic anhydride content, \(\sigma_p\), \(\sigma_u\), and \(\varepsilon\) increase. Moreover, as in the case of electrical strength, the listed characteristics have a maximum at an HET anhydride content of 20%.

Fig. 1. Dependence of the electrical and mechanical properties of the compound on the percentage content of chlorendic anhydride:
a) 1—“sphere—plane” electrodes; 2—“point—plane” electrodes; 3—tensile strength; 4—flexural strength; 5—relative elongation at break;
b) 6—relationship between the electrical and mechanical strength of the compound.

It is characteristic that, when comparing the electrical and mechanical strength of the formulations studied, one can note a tendency for the electrical strength to increase with increasing mechanical strength (Fig. 1,b).

Microscopic studies showed that in the compound containing 96 parts by weight of chlorendic anhydride there is a large number of gas pores, which remained in the rapidly curing compound after casting into molds. Obviously, the presence of gas pores was the main cause of the reduction in the electrical and mechanical strength of the compound at a high content of chlorendic anhydride.

It is known from the literature that, under prolonged exposure to an alternating voltage, dendrites begin to grow from the sharp corners of embedded metal electrodes and from the surface of ionizing gaseous inclusions—hollow tubes filled with gas and formed...

as a result of microscopic breakdowns of the dielectric in the region of local maximum electric-field intensities [3, 4]. The growth of dendrites leads to a decrease in long-term electrical strength and ends in breakdown of the insulation. We studied dendrite growth in point–plane electrodes by observing the moment of dendrite appearance and its growth through a special optical device. It was established that the addition of chlorendic anhydride changes the character and shape of the dendrites that form and leads to a significant increase in the dendrite resistance of the compound.

Thus, the addition of chlorendic anhydride to a cycloaliphatic resin accelerates the curing rate and improves the electrical and mechanical properties of the compound studied at room temperature.

References

1. V. A. Kargin, G. L. Slonimskii. Brief Essays on the Physical Chemistry of Polymers. Khimiya, 1967.

2. A. A. Tager. Physical Chemistry of Polymers. Goskhimizdat, 1963.

3. O. S. Pratt. Ann. Rep. NRC Conf. on Electr. Insulation, 1965, p. 74.

4. I. H. Mason. Lectures at Symposium on “Polymer as Electrical Insulation,” London, 1971.

5. Ch. Harper. Encapsulation of Electronic Equipment with Synthetic Resins. Energiya, 1964.

6. H. Lee, K. Neville. Handbook of Epoxy Resin, 1967.