JPS6246576B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a method for improving the properties of polyolefin sheet materials, and in particular provides a novel method capable of improving the voltage resistance performance of polyolefins. In general, plastic sheets or molded products are widely used for electrical insulation purposes. In recent years, with the demand for higher voltage equipment, there has been a demand for the development of plastics with superior dielectric strength. In particular, power cables, high voltage transformers, and capacitor equipment are increasingly required to have high performance from the standpoint of energy and resource conservation. For example, research is being carried out on new materials with excellent dielectric strength for the insulation of OF cables. That is, the development of composite insulating paper with high withstand voltage and low dielectric loss is underway to replace the conventional kraft paper insulation. One example is a sandwich structure of kraft paper and a layer of highly crystalline polyolefin. However, although this material is effective in improving dielectric strength, it has a limit in reducing loss. On the other hand, when an insulating layer is formed by laminating plastic films alone, swelling occurs in a short period of time, resulting in a significant decrease in dielectric strength. Even if a material that does not swell is found, it would be difficult to put it into practical use. Therefore, there is a need for a new material preferably made of plastic with low dielectric loss and with excellent withstand voltage. The inventors of the present invention have conducted intensive studies on the behavior of charged particles in molten plastics under high electric fields, and have found that the electric field acting near impurities becomes significantly larger than the applied external electric field.
When plastics containing impurities are placed under an alternating current electric field, a specific minute region centered on the impurities in the insulator is intensively exposed to a high electric field, which becomes a powerful trigger for alternating current breakdown. We have also found that these phenomena can be avoided by solidifying the polymer melt while processing it in advance under a DC electric field. The reason for this is not clear, but it is related to the boundary between the formation of the microstructure of the polymer and its electrical properties.Although it is difficult to find a clear causal relationship, it is probably due to the arrangement of impurities in the polymer solid that trigger electrical breakdown. This is thought to be caused by the fact that the potential becomes minimal with respect to the equipotential surface. In the method of the present invention, polyolefins include polypropylene, high density polyethylene, low density polyethylene, short chain low density polyethylene, polybutene.
It refers to homopolymers such as I, poly-4-methylpentene-I, or copolymers of ethylene and α-olefin, propylene, butene-I, oftene-I, and the like. Note that crosslinked products of these polymers are also included. In the method of the present invention, the polyolefin sheet is placed between electrodes and heated. The electric field strength applied at this time is 0.5 kV/mm or more, preferably 10 kV/mm or more, which is much lower than the breakdown voltage of polyolefin and close to the breakdown value of polyolefin in its molten state. The method of placing this molten polyolefin sheet under the electric field is not particularly limited; for example, it may be placed between a pair of parallel electrodes while being heated, or as a continuous method, for example, it may be melted between metal rolls. There are methods such as applying high voltage between rolls while guiding the film. The processing voltage is not particularly limited, and usually a DC voltage lower than the breakdown voltage of the melt is applied. In the method of the present invention, the polyolefin may be in the form of a film, a sheet, or a composite structure using these. For example, a laminated structure may be formed of a non-melting high dielectric constant sheet material such as kraft paper or a heat-resistant polymer film and a polyolefin film. Even in the case of such a composite structure, charged particles moving in the polyolefin are accumulated near the interface of the polyolefin layer, resulting in a structure integrated with a phase with a high dielectric constant such as kraft paper. The temperature of the polyolefin above the melting point or softening point is between the melting point or softening point and a temperature 50° C. higher than the melting point or softening point. The processing time cannot be determined because it depends on the thickness or structure of the object to be processed, such as a composite structure, but it is usually 5 to 50 minutes.
It is 120 seconds. Since the polyolefin treated by the method of the present invention has dielectric impurities that cause dielectric breakdown electrically removed, its electrical properties can be significantly improved. Next, examples of the method of the present invention will be described. Example 1 Polypropylene (No. 1), high-density polyethylene (No. 2), low-density polyethylene (No. 3), and linear low-density polyethylene (No. 4) were placed between two iron plates.
are each formed into a film using the heat press method, and the thickness
I got 120μ film. A circular electrode with a diameter of 20 mm was attached to each of these films, and a treatment voltage of 3.0 kV was applied to each film while holding it in the heating atmosphere shown in Table 1, dielectrically treating it for 70 seconds, and then cooling it to a high temperature to produce a polyolefin in film according to the method of the present invention. (product of the present invention) was obtained. For comparison with the method of the present invention, a comparative polyolefin film (comparative example product) was obtained in the same manner as in the example except that the polyolefin film described above was not subjected to any electrical treatment. The alternating current breakdown voltage at room temperature was measured for the thus obtained products treated in the present invention and the products treated in the comparative example. The results are also listed in Table 1.

[Table] As is clear from the above table, the electrical properties of the products treated according to the present invention are significantly improved. Example 2 Two 80 m/m diameter heating rolls (surface temperature 165°C), electrically insulated from each other and installed with a gap of 0.5 to 1.0 mm, were rotated at a surface speed of 180 cm/min. A cross-linked polyethylene film (1 m/m thick) with a gel fraction of 96% is passed through a cross-linked polyethylene film (1 m/m thick) with a DC electric field of 2.5 kV/mm applied between the rolls, and then solidified by a cooling roll and cross-linked by the method of the present invention. A polyethylene film (product treated according to the present invention) was obtained. In addition, for comparison with the method of the present invention, a crosslinked polyethylene film (comparative example treated product) was obtained by the comparative method in the same manner as in the above example without performing the above-mentioned electrification treatment. The alternating current breakdown voltage of the thus obtained treated products of the present invention and the comparative treated products was measured in insulating oil using a sphere with a diameter of 20 m/m and a flat electrode. This result is the second
As shown in the table.

[Table] Example 3 Using the heating roll used in Example 2, a composite insulating paper with 45μ thick cellulose paper thermally attached to both sides of a 30μ thick isotactic polypropylene film was charged under the following processing conditions. Processed. Roll gap 0.14mm, roll surface temperature 200℃, surface speed 180
A DC voltage of 300 V was applied between the rolls at a rate of cm/min, and then the resin of the composite insulating paper was immediately cooled and solidified using a cooling roll to obtain a composite insulating paper treated by the method of the present invention (product of the present invention). In addition, for comparison with the product of the present invention, a treated composite insulating paper (comparative example product) was obtained in the same manner as in the example except that the above-mentioned electrification treatment was not performed. AC breakdown voltages of the products of the present invention and comparative products thus obtained were measured in dodecylbenzene oil at room temperature. The results are shown in Table 3.

[Table] As detailed above, according to the method of the present invention, the specific withstand voltage of polyolefin can be dramatically improved, and in the case of polyolefin films, the drop in withstand voltage during lamination can be extremely minimized. Therefore, it is extremely useful as an insulating material for high-voltage electrical equipment.

Claims (1)

[Claims] 1. A method for improving the electrical properties of a polyolefin sheet material, which comprises cooling and solidifying a sheet material made of polyolefin heated to a temperature above its melting point or above its softening point under a predetermined direct current electric field.