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

Vol. 282 　　　　　　　　　　　　　　　　　　　　　　　　　　　　　1974

STUDY OF THERMAL EXPANSION OF SOME POLYMER FILMS

V. M. ANIKEENKO, T. V. KULIKOVA

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

Polymer films are widely used as electrical insulating materials in the manufacture of various electrical-engineering products and structures. At the same time, in each specific case the operation of the film has certain specific features, i.e., the films are subjected to various mechanical, electrical, and thermal effects. In all structures the polymer film operates at elevated

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Fig. 1. Dependence of thermal expansion \((\Delta l)\) and the coefficient of thermal expansion \((\alpha_l)\) of a polyethylene terephthalate film (lavsan) on temperature (in the transverse direction): 1 — \(\alpha_l\); 2 — \(\Delta l\) (in the initial state); 3 — \(\alpha_l\); 4 — \(\Delta l\) (upon repeated heating)

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Fig. 2. Change in \(\Delta l\) and \(\alpha_l\) as a function of the temperature of polyethylene terephthalate (in the longitudinal direction): 1 — \(\alpha_l\); 2 — \(\Delta l\) — initial state; 3 — \(\alpha_l\); 4 — \(\Delta l\) — after 24 hours of aging at \(140^\circ\text{C}\); 5 — \(\alpha_l\); 6 — \(\Delta l\) — after 240 hours of aging at \(140^\circ\text{C}\)

temperatures in contact with conductors and electrical insulating materials that have different coefficients of thermal expansion (CTE). In this connection, considerable internal stresses may additionally arise in the films, accompanied in a number of cases by deformations and damage in the form of cracks. In this regard, the study of the thermal expansion of films is of definite practical importance.

In the present work, a film with a linear and spatial molecular structure was studied. The objects of investigation used were Lavsan film, enamel films based on Viniflex varnish (VL-931), polyetheramide (PE-955), polyetherocyanurate (PE-958), and polyester (F-35).

Thermal expansion was studied by the dilatometric method. Existing designs of quartz dilatometers [1, 2] do not make it possible to record, with a sufficient degree of accuracy, the thermal expansion of films 30–60 μ thick; therefore, a universal instrument proposed in [3] was reproduced and used.

Thermal expansion was studied in the interval of positive temperatures from 20 to 200°C at a film-heating rate of 0.5–1°/min and was recorded with an MIR-1M microscope with a scale division value of 36 μ. Enamel-varnish films were prepared on substrates by the dipping method.

The character of the thermal expansion of a film of a linear crystalline polymer (polyethylene terephthalate) is shown in Figs. 1 and 2. For the experiments, specimens cut in two perpendicular directions, longitudinal and transverse, were used. As can be seen from Figs. 1 and 2, in the initial period of heating a slight expansion is observed, which subsequently turns into a sharp irreversible contraction. On repeated heating up to a temperature of 120–140°C, some expansion of the film is observed, and then, at higher temperatures, contraction. This behavior of Lavsan film is caused by the oriented arrangement of the molecules arising during manufacture. Forced orientation is characterized by a nonequilibrium state of the structure, which disappears in the process of molecular relaxation. Relaxation in this case is impeded by the crystalline structure and dense packing of the structure. In this connection, the nonequilibrium stressed state disappears only after 240 h of heat treatment at a temperature of 140°C, despite the fact that the glass-transition temperature lies in the inter-

Fig. 3. Change in $\Delta l$ and $\alpha_l$ as a function of temperature for PE-958 enamel-varnish films: 1 — $\alpha_l$; 2 — $\Delta l$ — unstructured film; 3 — $\alpha_l$; 4 — $\Delta l$ — structured film; 5 — $\alpha_l$; 6 — $\Delta l$ — the same upon repeated testing.

in the range 67–81°C [4]. This phenomenon should be taken into account in designing slot insulation for electrical machines, in manufacturing winding wires with film insulation, when compaction of the insulation is achieved by heat treatment, and also in studying the service life of polymer insulation. The change in the temperature coefficient of expansion ($\alpha_l$) has a more complex character, and it is sensitive to internal rearrangements and internal stresses.

In the process of producing enameled wires, an oriented arrangement of the molecules of the film-forming base arises when the varnish is applied to the wires.

Fig. 4. Dependence of $\Delta l$ and $\alpha_l$ on temperature for a PE-955 enamel-varnish film: 1 — $\alpha_l$; 2 — $\Delta l$ — initial state; 3 — $\alpha_l$; 4 — $\Delta l$ — after 24 hours of aging at 180°C; 5 — $\alpha_l$; 6 — $\Delta l$ — after 240 hours of aging at 180°C

Fig. 5. Dependence of $\Delta l$ and $\alpha_l$ on temperature for VL-931 enamel-varnish films: 1 — $\alpha_l$; 2 — $\Delta l$ — initial state; 3 — $\alpha_l$; 4 — $\Delta l$ — after 24 hours of aging at 180°C; 5 — $\alpha_l$; 6 — $\Delta l$ — after 72 hours of aging at 180°C

Stabilization of such a state is promoted by the formation of wall-adjacent layers at the metal–varnish interface [6, 5], by the structuring process, and by disorientation—relaxation processes during the period of solvent evaporation and the onset of polycondensation. During subsequent heat treatments the nonequilibrium state gradually decreases, and the level of internal stresses is reduced; this can explain the change in the magnitude of the TCE$_{\text{av}}$ jumps for PE-958 varnish films (Fig. 3), PE-955 (Fig. 4), and VL-931 (Fig. 5). The magnitude of the TCE for practically all enamel films in the temperature interval 20–200°C varies within the limits $(2 \div 20)\cdot 10^{-5}\ \text{1}/^\circ\text{C}$, which is approximately one order of magnitude higher than the TCE of metals. Such a difference in the temperature coefficients of expansion may lead to the occurrence of significant internal stresses. The magnitude of stresses in enamel insulation of wires and other products can be estimated with sufficient accuracy for the region of elastic deformations when the dependence $\alpha_l = f(t^\circ\text{C})$ and the elastic modulus $E = f(t^\circ\text{C})$ are known.

References

1. G. M. Barzhnov, V. I. Partsman. Zavodskaya laboratoriya, vol. 28, No. 21, 1962.

2. N. S. Novoseltsev. Practicum on Ferroelectrics. Rostov University, 1958.

3. I. F. Kaimin et al. Plasticheskie massy, No. 9, 1966.

4. Structural Properties of Plastics. Edited by E. Baer. Khimiya, 1967.

5. Yu. M. Malinskii. On the influence of a solid surface on relaxation and structure-formation processes in the wall-adjacent layers of polymers. Uspekhi khimii, vol. XXXIX, issue 8, 1970.

6. A. I. Kislov. Study of the electrical, adhesive, and physicomechanical properties of electrical-insulating varnish coatings under structural plasticization. Dissertation, 1968.