Study of the Influence of Extrusion Technology on the Properties of Polyethylene Wire Insulation

STUDY OF THE EFFECT OF EXTRUSION TECHNOLOGY ON THE PROPERTIES OF POLYETHYLENE WIRE INSULATION

V. M. Anikeenko, N. I. Frizen

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

The quality of the insulation of cable products made from thermoplastic polymers depends to a considerable degree on the extrusion technology. The stability of operation of the worm press, the method of extrusion, the temperature regime of polymer processing, and cooling are only some of the principal factors that affect the geometric dimensions and the physicomechanical and electrical-insulation properties of the insulation.

In the present work, changes in the product diameter and the influence of drawdown and of the number of filtering screens on the properties of the insulation were studied.

For the investigation, a wire with a single-wire conductor and polyethylene insulation with a diameter of 2.065 mm was used. Extrusion of the wire was carried out on a 24M-45 extruder under the following temperature regime: zone I—140–150°C, zone II—170–180°C, zone III—200–210°C, and the press head—210–215°C, followed by cooling with water at a temperature of 17–20°C.

Works [1, 2] point to continuous fluctuation of the wire diameter along its length. These fluctuations are random in character and are determined mainly by nonuniformity of the conductor speed, nonuniformity of press feeding with polymer, and the supply of melt to the head.

The wire diameter was measured with an accuracy of up to 0.2 μm. Statistical processing of the measurement results and evaluation of the distributions by Pearson’s goodness-of-fit criterion \(P(\chi^2)\) [3] showed that, under the given extrusion conditions, the character of the change in diameter follows the normal law. In the production of cable products, extrusion with drawdown is often used to increase press productivity. At the same time, abroad [4] and in domestic industry, various drawdown values are used without sufficient analysis of the influence of its magnitude on the physicomechanical and dielectric properties of the product insulation. Yet drawdown is accompanied by the appearance of considerable internal stresses, and the latter lead to a decrease in the electrical strength of the insulation [5].

In order to study the influence of drawdown on the properties of the insulation, wires were manufactured with drawdown \((B)\) equal to 1.0; 1.2; 1.5 and 2.0, the value of which was determined from the expression

\[ B = \frac{d_m^2 - d_{\mathrm{c}}^2}{d_{\mathrm{iz}}^2 - d_{\mathrm{c}}^2}. \]

where $d_m$, $d_{\mathrm{ins}}$, and $d_c$ are, respectively, the diameters of the die, the insulation, and the conductor.

During extrusion with draw-down, under the action of tensile and shear deformation, orientation of the polymer macromolecules occurs; subsequently it is partially reduced [5] because of the high temperature of the melt before the product is cooled. As a result, a certain anisotropy of the physicomechanical and dielectric properties of the insulation is created. Table 1 gives the values of the mechanical and electrical strength of the insulation at a 50% probability for different draw-down values.

The data in Table 1 indicate a certain increase in the mechanical ($\sigma_p$) and electrical strength ($E_{\mathrm{br}}$) of the insulation. At draw-down above 1.5 the characteristics practically do not change. Thus, increasing the draw-down to 1.5–2 improves $\sigma_p$ and $E_{\mathrm{br}}$ of the insulation.

As a result of the orientation of macromolecules during extrusion, internal stresses arise in the polymer and are frozen in. The magnitude of the latter increases with increasing draw-down, and the qualitative microscopic studies we carried out confirmed the presence of stresses in the insulation at the conductor surface. And if, for a conditional assessment of internal stresses, one takes the amount of shrinkage upon heating above the glass-transition temperature [6], then the experimental results indicate an increase in axial and radial shrinkage with the magnitude of draw-down. For a draw-down of 1, the axial shrinkage after 40 hours at a temperature of 80°C is 0.35%; at $B = 1.2$ the shrinkage is 0.38%; at $B = 1.5$ it is 0.41%; and at $B = 2$ it is about 0.43% and thereafter changes little.

Table 1

Consequently, on the basis of the change in the amount of shrinkage, it may be considered that the internal stresses partially relax and, in magnitude, differ only slightly when the draw-down is varied from 1 to 2.

Thus, in the production of low-voltage products, extrusion with a draw-down value within the above-mentioned limits is acceptable.

REFERENCES

1. I. M. Sandigursky. Automation of the process of manufacturing wires and cables with plastic insulation. Automation of control and regulation in the cable industry. Issue 1. Kishinev, 1966.

2. I. M. Sandigursky. System of automatic correction of the cable sheath diameter. Automation of control and regulation in the cable industry. Issue 2. Kishinev, 1966.

3. A. K. Mitropolsky. Technique of Statistical Computations. Moscow, Fizmatgiz, 1961.

4. A. Griff. Extrusion of Polymers. Moscow, IL, 1965.

5. V. S. Dmitrevsky. Calculation of Electrical Insulation. Tomsk, TPI, 1971.

6. Chizhov et al. Resistance to cracking of extruded polyethylene. Plasticheskie massy, 1967, No. 10.