FROST RESISTANCE AND HEAT RESISTANCE OF INSULATING COMPOUNDS

N. I. VOROB'EV

Classification of Insulating Compounds

(Presented by Prof. A. A. Vorob'ev, Doctor of Physico-Mathematical Sciences)

The use of insulating compounds (insulating masses or compounds) for the purpose of potting and impregnating the insulation of electrical machines and apparatus is widely practiced in modern electrical-insulation technology. At the present time, insulating compounds with increased heat resistance or high frost resistance, or both at the same time, i.e., with an increased range of operating temperatures, are especially necessary. Despite the large number of existing formulations of insulating compounds that partially or fully satisfy the stated requirement, the theoretical questions of the frost resistance and heat resistance of insulating compounds have been poorly studied. In the present work, on the basis of known regularities established for the frost resistance and heat resistance of high-molecular compounds in general, an attempt is made to develop the questions of frost resistance and heat resistance of insulating compounds.

Depending on their purpose, insulating compounds may be divided into 3 types: impregnating, potting, and coating.

The regularities considered in the present work pertain mainly to potting and coating compounds, but they are also partially valid for impregnating compounds and varnishes.

Depending on their composition, insulating masses may be divided into several groups:

1) masses based on low-molecular compounds: wax-like and bituminous;

2) masses based on high-molecular compounds: based on polymerization-type resins and based on condensation-type resins. Masses of mixed type are also encountered.

The indicated insulating masses are characterized by the following features:

1. Wax-like masses have reduced adhesion, low viscosity in the molten state, a low potting or impregnation temperature, and some brittleness at room temperature.

2. Bituminous masses possess high adhesion, an increased potting or impregnation temperature as a result of increased viscosity, and low cost.

3. For masses of the condensation or polymerization type, an increased value of the range of operating temperatures is characteristic. In their initial form these are liquids, the hardening of which is carried out by heating.

Depending on the behavior of insulating compounds, i.e., their relation to the action of temperature, solvents, etc., the compounds may be sub-

…to divide, similarly to the classification of high-molecular compounds, into thermoplastic and thermosetting. The principal properties of thermoplastic and thermosetting compounds are given in Table 1.

Table 1

Frost Resistance of Insulating Compounds

Let us imagine some insulating part placed in a casing, most often metallic, and filled with a compound. When such a structure is cooled, thermal deformation of all its parts occurs; because of the difference in the values of the coefficients of linear expansion of the metal and of the compound, the casing contracts more slowly than the casting compound. This leads to the appearance of internal stresses in the mass of the compound, which, under sufficiently strong cooling, causes cracking of the compound. The temperature at which cracks appear characterizes the frost resistance of the given compound.

For simple amorphous bodies (for example, rosin, bitumen, etc.), frost resistance is determined by the glass-transition temperature, i.e., the temperature at which the viscosity of the substance reaches \(10^{13}\) poise. At lower viscosities, the internal stresses that arise have time to relax as a result of the viscous flow of the material. When the viscosity rises to \(10^{13}\) poise, stress relaxation does not occur, and the amorphous body cracks under the application of the slightest load [7].

For high-molecular substances, frost resistance also depends on the glass-transition temperature, but does not coincide with it completely. For high polymers (thermoplastics), the glass-transition temperature is defined as the temperature at which the high-elastic properties of the polymer are lost. In view of the great importance of the high-elastic deformation of polymers for their frost resistance, let us consider this phenomenon in somewhat greater detail.

According to modern views, molecules of high polymers are long, flexible chains. The flexibility of the molecule determines the elasticity of the polymer and, in turn, is due to the fact that individual units of the molecule can rotate around the C—C bond, as around an axis.

while maintaining the constancy of the valence angles and interatomic distances. Under normal conditions (without the application of an external force), the molecule is coiled into a ball. When an external force is applied, it straightens, which accounts for the high elasticity of the polymer.

The degree of flexibility of the molecule depends on the strength of the interaction of the segments with one another [8], which is greater for polar radicals (segments).

The greater the elasticity of the polymer at room temperature, the longer it retains its highly elastic properties upon cooling, and the lower its glass-transition temperature lies [9]. Consequently, as the polarity of the polymer decreases, its glass-transition temperature should decrease. This conclusion is well confirmed by Table 2, in which polymers (thermoplastics) of different degrees of polarity are grouped. The data for Table 2 are taken from various literature sources.

Table 2

The indicated regularity is also valid for substances of low molecular weight (bitumens, rosin, etc.). In this case, a new dependence is added: with a decrease in the molecular weight of the substance, the glass-transition temperature shifts somewhat toward lower temperatures. For example, Fig. 1 gives the dependence of the glass-transition temperature of polyisobutylene on molecular weight (curve $T_c$) [10].

Simple amorphous bodies crack at the glass-transition temperature when even the slightest mechanical load is applied to them. In contrast, high polymers withstand a mechanical load and give large elongation at temperatures considerably lower than the glass-transition temperature [11]. The large deformations (“forced elasticity,” according to Lazurkin and Fogelson) of polymers below the glass-transition temperature are explained by the increase in the rate of rearrangement of individual segments of molecules (i.e., by the increase in the rate of relaxation processes) with the growth of internal mechanical stresses. As a result, highly elastic properties return to the polymer.

During deformation of a polymer, stretching and orientation of macromolecules occur in the direction of deformation. This produces the effect of strengthening the polymer [7].

The presence of large deformations and an increase in the mechanical strength of the polymer lead to the fact that the brittleness temperature of the polymer, i.e., the temperature at which only elastic deformation appears at the moment of rupture and forced elasticity is absent, lies considerably below the glass-transition temperature; that is, the frost resistance of the polymer is substantially increased. Figure 1 shows the dependence of the brittleness temperature of polyisobutylene on molecular weight (curve \(T_{\mathrm{br}}\), solid line) [12]. The brittleness temperature was determined under rapid deformation.

Fig. 1.

It seems to us that the above-mentioned regularities, established for high-molecular compounds, are also applicable to insulating compounds. In this case, a correction must be introduced for the curve \(T_{\mathrm{br}}\) (Fig. 1). In the service of casting and coating compounds, the load is applied extremely slowly (temperature deformation during cooling), and therefore the brittleness temperature will lie considerably lower—somewhere in the region of the dashed curve.

Thus, owing to the presence of forced elasticity, the frost resistance of thermosetting (i.e., in other words, high-molecular) insulating compounds, in comparison with the frost resistance of thermoplastic compounds, is substantially increased.

The frost resistance of an insulating compound can be increased by introducing a plasticizer, i.e., a substance with a lower glass-transition temperature. The role of the plasticizer is reduced to separating the polymer molecules,

toward reducing their interaction, which increases the elasticity of the substance (in the case of simple amorphous bodies it lowers the viscosity), i.e., lowers the glass-transition temperature.

Wax-like (crystalline) additives may also play the role of a plasticizer. Waxes introduced in small amounts become supercooled together with the principal mass of the compound; moreover, the glass-transition temperature of the supercooled wax will lie considerably below its melting point \([7;13]\), i.e., the wax will behave as a plasticizer. With a larger introduction of wax-like material, supercooling of the wax will not occur, and the insulating compound will consist of a crystalline and an amorphous phase.

Heat Resistance of Insulating Compounds

The heat resistance of insulating compounds, characterizing the maximum permissible operating temperature, is determined differently for different types of compounds. The heat resistance of thermoplastic insulating compounds is limited by their softening temperature. For thermosetting compounds, heat resistance is determined by the highest temperature at which no thermal aging of the compound is yet observed: inadmissible decrease in elasticity, swelling, etc.

To establish the softening temperature of bituminous and wax-like compounds, the ring-and-ball and Ubbelohde methods are often used. For thermoplastic polymers the softening temperature may be defined as the temperature at which flow of the polymer begins.

The softening temperature depends to a great extent on the polarity and molecular weight of the polymer. With increasing polarity of a substance, the interaction of polar groups increases, which is reflected in a rise in the softening temperature. The increase in the softening temperature for polar polymers is clearly seen from Table 2, the data for which, as already indicated, were selected from various literature sources. The polymers had approximately the same molecular weight.

Thus, with increasing polarity of the material, both the softening temperature \(T_p\) and the glass-transition temperature \(T_c\) of the material increase. The softening interval, equal to the difference between these temperatures \((T_p - T_c)\), should not change for materials of different degrees of polarity (other conditions being equal, for example, at the same molecular weight). This regularity is also evident from Table 2.

As already indicated, the glass-transition temperature of a polymer can be lowered by introducing a plasticizer. But because the mechanism of action of the plasticizer is the same both in glass transition and in softening of the polymer, simultaneously with the lowering of the glass-transition temperature there occurs a lowering of the softening temperature of the polymer \([14]\).

Applying the indicated rule to insulating compounds, it should be concluded that the introduction of a plasticizer should not increase the softening interval of the insulating compound \((T_p - T_c)\), and consequently not the interval of operating temperatures either, but merely shifts them toward lower temperatures. This conclusion is confirmed by the work of Baev \([15]\) for bituminous compounds.

For insulating compounds of comparatively low molecular weight (bituminous and wax-like), the softening interval is small and does not exceed \(130—180^\circ\mathrm{C}\). An entirely different phenomenon is observed for high-molecular insulating compounds. It is known that in polymers (thermoplastics), above the glass-transition temperature there is a broad region of high-elastic deformation—a state ideal for insulating compounds—and only after it does polymer flow begin. With an increase in the molecular weight of the polymer, the width of the region of high-elastic deforma-

expands; consequently, the softening temperature of the polymer rises, as is evident from Fig. 1, which shows the dependence of the softening temperature (curve \(T_p\)) of polyisobutylene on molecular weight [16].

Therefore polymers have, in comparison with low-molecular substances, an increased range of operating temperatures both because of the increased value of the softening temperature \(T_p\) and because of the broadening of the interval down to the brittleness temperature \(T_{\mathrm{br}}\). Even a small introduction of high-molecular additives increases the softening interval of an insulating compound [3].

In the case of transition from linear polymers to three-dimensional ones, with a small number of cross-linking bridges, the elasticity of three-dimensional polymers decreases only slightly, and consequently the brittleness temperature and frost resistance change little. At the same time, heat resistance not only does not decrease, but may be increased by eliminating the possibility of softening. As a result, thermosetting insulating compounds, in comparison with bituminous and wax-like ones, have a considerably larger range of operating temperatures.

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