EFFECT OF HEAT TREATMENT ON THE PROPERTIES AND STRUCTURE OF POLYETHYLENE

M. N. TRESKINA, G. I. SYCHEV, A. N. RUDNEV

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

The application of polyethylene insulation to power cables is carried out on screw extruders at a temperature of 180–210°C. Taking into account the considerable shrinkage of polyethylene and the formation of pores in the insulation, cooling of the insulated product is performed in baths with specified temperatures: in the first bath 95–85°C, in the second bath 75–65°C, in the third bath 50–40°C, and final cooling at the temperature of running water. Depending on the length of the baths, the line speed of insulation, and the thickness of the applied insulation, the cooling rate varies over wide limits.

A characteristic feature of the structure of polyethylene is the combination of crystalline and amorphous phases: high-pressure polyethylene at 20°C contains 70–75% crystalline phase. The quantitative ratio between the crystalline and amorphous phases in polyethylene depends on the nature of the chains (length and branching), on the temperature of the polyethylene, and on the processing technology. Many of its physicochemical and mechanical properties depend on the quantitative ratio of crystalline and amorphous phases in polyethylene.

Studies of the structure of polymers have shown that, during formation of the crystalline phase, several forms of supramolecular structure may occur [1]. In the amorphous state, polyethylene is characterized by globular and bundle-like supramolecular structures; crystallization may proceed by both “plate-like” and fibrillar mechanisms; crystalline formations in polyethylene have a spherulitic structure. The presence of many forms of supramolecular structure, their mutual transformability, growth and destruction, and quantitative ratio exert a decisive influence on the properties of crystalline polymers.

Changing the cooling rate of polyethylene insulation leads to a change in the number and size of pores in the insulation and to a change in the crystallization regime of polyethylene. Both the former and the latter will affect the properties of the cable insulation.

In the present work, studies were carried out to determine the influence of the cooling rate of polyethylene on a number of electrical and mechanical properties and on the features of the structure of polyethylene.

At this stage, the work was performed on specimens of unstabilized Salavat high-pressure polyethylene. Plates 0.5 mm and 2.0 mm thick were made from granulated polyethylene.

on laboratory rolls with subsequent pressing in a hydraulic vulcanization press at a temperature of 143°C and a pressure of 10 kg/cm² for 10 min. The cooling rate after pressing was 1°C per minute for one batch of specimens, and 10°C per minute for the other.

In this work, temperature–time dependences of the electrical conductivity of polyethylene specimens were obtained. Currents were measured on a galvanometric setup using a vacuum-tube electrometer with a sensitivity of \(10^{-14}\) A; the currents were recorded on an electronic potentiometer of the EPPV-51M type. According to the data obtained, polyethylene is characterized by a considerable decrease in current with time after the application of a dc voltage at field strengths of 0.2–0.5 kV/mm in the temperature range from \(+20^\circ\)C to \(+70^\circ\)C.

Figure 1 shows the dependence of the specific volume resistivity \(\rho_v\) of rapidly cooled (B) and slowly cooled (M) polyethylene specimens on temperature after holding the specimen under voltage for 1 min. In the dependence \(\lg \rho_v = f(t^\circ\mathrm{C})\), a maximum is observed: for specimens B it lies in the temperature region 50–60°C, and for specimens M at a temperature of 70°C. The obtained values of \(\rho_v\) in the temperature range 20–70°C characterize the polarized state of the specimens. Moreover, with increasing temperature the absorption currents decrease and the calculated values of \(\rho_v\) increase. The decrease in \(\rho_v\) at temperatures above 50–70°C is associated with an increase in impurity conductivity during the thermal expansion of polyethylene. The position of the maximum of \(\rho_v\) possibly corresponds to the temperature region of the greatest thermal expansion of polyethylene, caused by the rapid transition of the crystalline phase into the amorphous phase. Then, according to Fig. 1, such a transition in rapidly cooled polyethylene occurs at lower temperatures than in slowly cooled polyethylene.

Fig. 1. Dependence of \(\lg \rho_v\) on temperature for rapidly (B) and slowly (M) cooled polyethylene specimens. \(\rho_v\) was calculated from currents after holding the specimen under voltage for one minute. Specimen thicknesses: 0.5 mm (1) and 2.0 mm (2).

Measurements of \(\operatorname{tg}\delta\) were carried out on an R-525 bridge at a voltage of 1 kV and industrial frequency. As can be seen from Fig. 2, with increasing temperature from 20 to 80°C there is a decrease (almost by an order of magnitude) in \(\operatorname{tg}\delta\) of specimens B and M, which may be associated with a decrease in low-frequency relaxation according to [2, 3]. According to [3], low-frequency relaxation is caused by the motion of dipoles in crystalline regions.

The dielectric permittivity and \(\operatorname{tg}\delta\) of slowly cooled polyethylene are higher than \(\varepsilon\) and \(\operatorname{tg}\delta\) of rapidly cooled polyethylene (Fig. 2), which can be explained by the larger number of carbonyl groups formed-

formed during oxidation of the polyethylene chains in the process of slow cooling, or to a greater degree of crystallinity according to the concepts of [3].

Heat treatment has a substantial effect on the magnitude of the crystalline phase—that is, on the degree of crystallinity. Quenching of polyethylene and polytetrafluoroethylene leads to an almost complete absence of the crystalline phase, whereas annealing causes an increase in the crystallinity of polymers compared with the initial material [4]. Therefore, a decrease in the cooling rate of polyethylene will lead to an increase in the degree of crystallinity, which is possibly one of the reasons for the increase in \(\varepsilon\) and \(\tg\delta\) of specimens M compared with \(\varepsilon\) and \(\tg\delta\) of specimens B.

Fig. 2. Dependence of \(\tg\delta\) and \(\varepsilon\) on temperature for polyethylene specimens subjected to rapid (B) and slow (M) cooling

Polyethylene specimens were subjected to breakdown under brief exposure to an electric field of industrial frequency with needle–plane electrodes. By means of a micrometer screw the needle was inserted into specimens 0.5 mm thick by 2.0 mm, ensuring a thickness of the punctured polyethylene layer equal to 0.3 mm. The electrode system with the specimen was placed in transformer oil.

Fig. 3 shows the integral distribution curves of the breakdown-voltage values of polyethylene specimens subjected to rapid and slow cooling. The mean value \(U_{\text{br}}\) for specimens M is \(\approx 10\%\) higher than \(U_{\text{br}}\) for specimens B. Polyethylene subjected to rapid cooling is characterized by a larger

Fig. 3. Integral distribution curve of breakdown-voltage values for polyethylene specimens subjected to rapid (B) and slow (M) cooling

scatter of the values of \(U_{\mathrm{br}}\) and, consequently, a lower homogeneity of the insulating layer in comparison with the slowly cooled specimens.

The study of the mechanical properties of polyethylene showed (Fig. 4) that the mean value of the tensile strength of slowly cooled specimens is 20% higher than the tensile strength of rapidly cooled specimens. During prolonged cooling of polyethylene, the crystallization process is slowed, which leads to a reduction in the possibilities for creating large internal mechanical stresses in the crystallizing polymer. In addition, prolonged holding also leads to a weakening of the existing stresses as a result of their relaxation. The reduction of the stresses arising during crystallization leads to progress of the crystallization process toward the establishment of equilibrium in the system and, as a consequence, to an increase in the homogeneity of the system [1]. The increase in the tensile strength of specimens M, as compared with the strength of specimens B, is associated with the increase in the crystallinity of polyethylene and the reduction of internal stresses. The increase in the tensile strength of slowly cooled polyethylene specimens corresponds to a fivefold decrease in relative elongation in comparison with rapidly cooled specimens.

Fig. 4. Integral distribution curves of the values of tensile strength (a) and relative elongation (b) of rapidly (B) and slowly (M) cooled polyethylene.

The residual elongation of specimens M is 40%, whereas for specimens B it is above 160%. The experimental data on the values of relative and residual elongations indicate an increase in the rigidity of polyethylene as the rate of its cooling is decreased.

The formation, during crystallization, of well-formed fibrils leads to an increase in strength and elongation at break. Elastic properties are lost only when coarse-crystalline structures are formed—primarily spherulites [1]. Thus, the change in the mechanical properties of polyethylene with a change in its cooling rate is associated not only with the size of the crystalline formations (with the degree of crystallinity), but also with the nature of the supramolecular structures.

To assess the change in the structure of polyethylene at different cooling rates, the structure of the specimens was studied in this work by means of an electron microscope.

Polyethylene specimens were frozen in liquid nitrogen and fractured. After the specimen was “thawed” and dried in a desiccator, a replica was made from the fracture surface of the polyethylene; for this purpose, a carbon-platinum film, or a carbon film shadowed with chromium, was deposited in a VUP unit. The replica was removed from the fracture surface with the aid of gelatin. After the gelatin was dissolved and the replica dried, it was examined in an electron microscope in the laboratory of the Department of Theoretical Electronics of TPI.

Figure 5 shows the structural patterns characteristic of rapidly cooled polyethylene (a) and slowly cooled polyethylene (b), obtained from carbon-platinum replicas. Figure 6 shows structural patterns

a                  b

Fig. 5. Structures of polyethylene after rapid cooling (a) and slow cooling (b), obtained from carbon-platinum replicas

of polyethylene obtained from carbon replicas shadowed with chromium. As can be seen from Figs. 5 and 6, changing the cooling rate of polyethylene from 1 to 10°C per minute causes a qualitative change in its structure.

Moreover, slowly cooled polyethylene is characterized by a more highly branched and ordered spherulitic structure (Figs. 5b and 6b) in comparison with the structure of rapidly cooled polyethylene (Figs. 5a and 6a).

Thus, a decrease in the cooling rate of polyethylene is accompanied by an increase in the degree of crystallinity and by a change in the character of the structure: the orderliness and branching of the spherulitic structure increase.

On the basis of the results obtained, the following conclusions may be drawn:

1. A change in the cooling rate of polyethylene from 1 to 10°C per minute leads to a change in the electrical and mechanical properties and in the character of the supramolecular structure.

2. In the temperature dependence of $\rho_v$, calculated from the currents measured after a one-minute holding of the specimen under a direct-current voltage, a maximum is observed; for slowly cooled specimens this maximum is shifted toward higher temperatures.

3. Slowly cooled polyethylene is characterized by higher values of $\operatorname{tg}\delta$, $\varepsilon$, and $U_{\text{pr}}$ in comparison with rapidly cooled specimens. The character of the integral distribution curves of $U_{\text{pr}}$ indicates the greater uniformity of polyethylene in specimens M.

4. A decrease in the cooling rate from 10 to 1°C per minute causes a 20% increase in the tensile strength of polyethylene, a fivefold decrease in the relative elongation, and a more than 4–5-fold decrease in the residual elongation.

5. A decrease in the cooling rate of polyethylene, in addition to increasing the degree of crystallinity, leads to a change in the character of the structure: the orderliness and branching of the spherulitic structure increase.

REFERENCES

1. V. A. Kargin, G. L. Slonimskii. Short Essays on the Physical Chemistry of Polymers. Izd. “Khimiya,” 1967.
2. B. I. Sazhin. Electrical Conductivity of Polymers. Izd. “Khimiya,” 1964.
3. G. P. Mikhailov, S. P. Kabin, B. I. Sazhin. ZhTF, 25, 590, 1955.
4. R. Houwink, A. Staverman. Chemistry and Technology of Polymers, vol. 1. Izd. “Khimiya,” 1965.

Fig. 6. Structures of rapidly cooled (a) and slowly cooled (b) polyethylene, obtained from carbon replicas shadowed with chromium.