On the Deaeration of Granular Polyethylene During Pressing

V. G. Sotnikov

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

At present it is well known that the presence of pores in high-voltage insulation made of polymers leads to its accelerated destruction due to ionization processes. In addition, a large number of pores leads to a reduction in the thickness of the insulating material and, consequently, to a reduction in the breakdown voltage of the high-voltage structure.

Large thick-walled articles are usually manufactured by pressing. In this case the air, which occupies 40–45% of the volume filled with granular polyethylene, may remain within the volume of the article. Depending on the pressing technology, it remains in the form of inclusions of various sizes or passes into a dissolved state. However, the transition of air into the dissolved state is also undesirable, since oxygen entering the volume of the polyethylene causes its accelerated aging and a reduction in its electrical characteristics [1, 2].

The irregular shape of the granules, their movement during melting, and the consequent possibility of coalescence of air inclusions do not make it possible to determine the largest dimensions of cavities in the casting. Experimentally, however, it has been found that, in the manufacture of cylindrical blanks 110 mm in diameter and weighing 800 g by fusion of low-density polyethylene granules at atmospheric pressure, the size of air inclusions reaches 4 mm.

The possibility of the formation of such large air inclusions in large articles operating as high-voltage insulation is inadmissible and makes it necessary to carry out deaeration of granular polyethylene during its melting.

According to published data [3], deaeration of polyethylene in a press mold can be carried out by a pressure exceeding its yield point at the given temperature. Moreover, the optimum deaeration temperature is considered to be 80–90°C. However, these data have no experimental confirmation regarding the escape of air from the volume of the press mold, and there is no estimate of the amount of air displaced from the volume of the press mold.

We measured the amount of air displaced from the volume of the press mold during deaeration of granular polyethylene by pressure using the apparatus shown in Fig. 1.

Granular polyethylene in an amount of 580 g was poured into press mold (1), which had an internal diameter of 100 mm, into which

Fig. 1. Apparatus for measuring the amount of air displaced from granular polyethylene under pressure

Fig. 2. Dependence of the amount of air remaining in the volume of the press mold (reduced to normal pressure) on pressure at \(t = 23^\circ\mathrm{C}\)

then the plunger with washer (2) was inserted. To eliminate the possibility of air escaping into the gaps between the plunger and the cylinder, and between the cylinder and the bottom of the mold, these were sealed with gaskets made of silicone rubber (3). Air from the volume of the mold passed through slotted channels of cross section \(2 \times 7\ \mathrm{mm}^2\) in the washer (2) into the cavity of the plunger, from where it entered, through a rubber tube (4), the measuring part of the apparatus, which consisted of a system of two communicating vessels (5 and 6) filled with water.

The measuring system was connected to the branch pipe of the mold only after the plunger with the washer had been inserted into the latter and had descended to the surface of the polyethylene.

The amount of displaced air was determined from the difference in water levels (\(BC\)) in vessel (6). For this purpose, before compression of the polyethylene in the mold, the water level in vessel (5) was marked; at that time it coincided with the water level in vessel (6) (point \(B\)). As the air was displaced from the working volume of the mold under pressure, its level in vessel (6) changed. In order that the displaced air should be at atmospheric pressure, vessel (6) was lowered, thereby ensuring the same water level in both vessels. The difference in levels \(BC\) was read after holding the mold for 5 min at the specified pressure on the plunger.

Figure 2 shows the dependence of the amount of air remaining in the volume of the mold on the specific pressure. It follows from this figure that at a temperature of \(23^\circ\mathrm{C}\) the amount of air in the working volume of the mold decreases by 55% when the pressure is increased to \(98 \cdot 10^4\ \mathrm{N}/\mathrm{m}^2\) (\(100\ \mathrm{kgf}/\mathrm{cm}^2\)). At higher pressures, up to \(294 \cdot 10^4\ \mathrm{N}/\mathrm{m}^2\) (\(300\ \mathrm{kgf}/\mathrm{cm}^2\)), the volume of displaced air changes only slightly. At the same time, measurements at an elevated temperature of the granular polyethylene—\(70^\circ\mathrm{C}\)—and a pressure of \(294 \cdot 10^4\ \mathrm{N}/\mathrm{m}^2\) showed that the volume of displaced air is almost 9% less than at normal temperature.

It should be noted that, in this experiment, the conditions for air escape were somewhat better than in ordinary molds, since the cross section of the special slotted channels is much greater than that of the gaps between the parts of the mold.

The weak influence of pressure and temperature on the amount of displaced air is probably due to the fact that, as the pressure is increased, the slotted channels and the gaps between the parts of the mold become blocked by polyethylene, and this process is facilitated by an increase in temperature.

Conclusions

1. From the data presented it follows that deaeration of granular polyethylene by pressure does not ensure complete removal of air from the working volume of the mold. The amount of air remaining in this case reaches 45–47% of the initial volume.

2. The temperature of the granular polyethylene has practically no effect on the amount of air remaining during deaeration by pressure.

3. The volume of air inclusions in a product can be reduced by approximately a factor of 2 by deaeration under pressure and subsequent melting and cooling without pressure. In this connection, vacuum deaeration of granular polyethylene during melting should be considered more effective.

References

1. V. S. Shifrin, N. N. Samosatskii. Polyethylene. Goskhimizdat, 1961.

2. N. Grassi. Chemistry of the Processes of Polymer Destruction. Foreign Literature Publishing House, Moscow, 1959.

3. V. S. Shifrin, N. N. Samosatskii. Polyethylene. (Processing and Application), GNTI Khim. lit., Leningrad, 1961.