KR100236782B1 - Resin for controlling charge distribution - Google Patents

Abstract

The present invention relates to a resin capable of controlling the charge distribution present in an insulator under a high voltage. The resin is a copolymer grafted onto polyethylene in which an acrylic acid having a polar group is an insulator. According to the graft ratio, the polarity and the amount of charge accumulated can be adjusted, which can be usefully used for improving the performance of the insulator of the power cable.

Description

Resin and power cable for charge distribution control

The present invention relates to a resin for controlling charge distribution made by graft reaction of acrylic acid with a monomer capable of controlling charge distribution on a polyethylene base resin used in underground cable power cables, and a method of manufacturing the same.

Generally, power cables for underground lines include external insulation shields, which include conductors, conductor strands, inner shields, inner semiconductive layers, main insulation layers, and neutral layers. outer semiconductive layer, watertight layer, and sheath, jacket.

When a power cable having such a structure is laid underground and placed in a long time use environment, the insulator is in a high electric field. It is well known that space charges are injected from the electrodes in an insulator placed in an electric field, and that the accumulation of space charges for a long time can seriously affect the life of the insulating material (see Mizutani, “Space Charge Measurement Techniques and Space Charge in Polyethylene ”, IEEE Trans, Dielectr.Electr.Ins., Vol. 1, pp. 923-933, 1994).

The role of space charge in high voltage power cables is very important. Therefore, in the case of direct current, electric charges stored in the insulator may cause dielectric failure due to instantaneous polarity reversal caused by lightning strike or repetitive switching surge. There is.

In the case of alternating current, a tree, which is a minute crack, is generated by space charges accumulated around protrusions, voids, or impurities, and leads to dielectric breakdown according to the progress of the tree. It is greatly affected by space charge. At this time, the electrons get very high energy due to repetitive change of polarity, so that they become hot electrons, and the hot electrons also promote deterioration of the insulator.

Therefore, in order to secure the reliability of the insulating material by extending the insulating life of the insulating material, it should be possible to prevent the charge accumulation or at least adjust the shape of the charge accumulation.

Polyethylene, an insulator used in conventional power cables, is a material in which ethylene is connected in the form of a continuous chain and is nonpolar, and thus has no ability to quickly remove charges injected from the outside.

Therefore, in order to control the charge distribution of the polyethylene, it is required to move the injected charge out of the insulator correctly or to distribute it evenly in the material, that is, to distribute the charge evenly.

For this purpose, generally known research has been carried out through various methods, and it has been reported that the improvement of the insulating property through the control of the charge distribution by blending the ethylene copolymer (see KS Suh, JY Kim, CR Lee, T. Takada, “Charge Distribution in Polyethylene / Ethylene Vinylacetate Laminates and Blends”, IEEE Trans. Dielectr.Electr.Ins., Vol. 3, pp. 201-206, 1996).

However, blends in ultra-high voltage cables are very likely to deteriorate the electrical properties due to the mixed additive itself as an impurity to improve the physical properties, phase separation phenomenon due to incompatibility, deterioration of mechanical properties, additives and Compatibility, or charge accumulation at the interface between components (N. Hozumi, T. Okamoto, and T. Imajo, “Space Charge Accumulation and decay at the Interface between Polyethylene and Ethylene-Vinylacetate Copolymer). ”, Proc. 8th ISH, Yokohama, Japan, pp. 111-114, 1993).

In order to solve the above-mentioned drawbacks, the present inventors have made the assumption that modification by graft chemically bonding monomers having functional groups to be mixed to the main chain of polyethylene will be more effective. Based on the research, the present invention was completed.

1 is a view schematically showing a charge distribution measuring apparatus for measuring the charge distribution of the charge distribution controllable resin composition according to the Examples and Comparative Examples of the present invention.

2 is a view showing a charge distribution measurement result of acrylic acid-grafted polyethylene according to Examples 1 to 5 of the present invention in which acrylic acid was grafted at different ratios in the main chain of polyethylene and the charge distribution of polyethylene.

* Explanation of symbols for main parts of the drawings

1: aluminum electrode 2: semiconductor electrode

3: PVDF film 4: aluminum foil

10: high voltage generator 20: electric pulse generator

30: specimen holder 40: measuring device

50: amplifier 60: oscilloscope

70: computer

According to the present invention, a resin having a polar functional group is grafted onto polyethylene, which is an insulator, to provide a resin capable of controlling the charge distribution present in the insulator under high voltage.

More specifically, according to the present invention, a resin for charge distribution control is provided, wherein acrylic acid as a monomer is grafted to the main chain of the low density polyethylene resin.

Hereinafter, the present invention will be described in more detail.

In the resin for charge distribution control of the present invention, polyethylene, which is a basic resin, is preferably a low density polyethylene used for main insulation of power cables.

The charge distribution control resin according to a preferred embodiment of the present invention may be prepared by grafting the main chain acrylic acid of polyethylene at low density by adding 5 parts by weight or less of acrylic acid and 0.001 to 1 part by weight of organic peroxide to 100 parts by weight of the low density polyethylene resin. have.

Acrylic acid grafted to the main chain of polyethylene can express the charge distribution control performance even with a very small amount, so the addition amount is not limited to the lower limit, but the upper limit is 5 parts by weight based on 100 parts by weight of polyethylene. It is desirable not to exceed. If the content of acrylic acid is more than 5 parts by weight, the content of unreacted acrylic acid is increased, thereby making it difficult to control the final properties. In addition, at 5 parts by weight or more, it is not preferable because the problem of controlling the desired charge distribution may occur. If this is expressed in terms of the ratio (grafting rate) of acrylic acid grafted to the low density polyethylene, it is preferable to make the graft rate determined by the following formula be 0.20 wt% or less. If the graft rate is 0.20wt% or more, the negative charge increases and there is a concern that the dielectric loss may increase greatly due to the increase of the polar acrylic acid content. Therefore, the graft rate of acrylic acid in the composition is advantageously adjusted to 0.20 wt% or less.

[Equation 1]

The initiator used to graft the acrylic acid to the main chain of polyethylene is preferably an organic peroxide initiator such as dicumyl peroxide. Such an organic peroxide-based initiator acts on polyethylene and acrylic acid as monomers to graf the acrylic acid to polyethylene. The amount of the organic peroxide initiator added for the graft reaction is preferably 0.001 to 1 parts by weight based on 100 parts by weight of the polyethylene resin. When the added amount of the organic peroxide initiator is less than 0.001 part by weight with respect to 100 parts by weight of the polyethylene resin, the graft effect is insignificant, and when it exceeds 1 part by weight, the graft reaction and the crosslinking reaction are likely to occur together, and simultaneously with the graft reaction. When the crosslinking reaction occurs, it is difficult to achieve the object of the present invention of controlling charge distribution by reducing the homogeneity of the resin.

Hereinafter, the present invention will be described in more detail with reference to Examples, but the present invention is not limited to Examples.

In the following examples, a charge distribution measuring apparatus using a pulsed electroacoustic method (PEA) was used to measure the charge distribution. The schematic diagram is the same as FIG. The structure of the charge distribution measuring device and a method of measuring the charge distribution using the same will be described briefly as follows.

The charge distribution measuring apparatus of FIG. 1 includes a high voltage generator 10, an electric pulse generator 20, a specimen holder 30, a detector 40, an amplifier 50, and an oscilloscope 60. And a computer 70.

When a 10 to 40 kV / mm direct current electric field is applied to the electrode of the device for 30 minutes on the sheet, the charge accumulates inside the specimen. When an electrical pulse of 10 nsec in width and -2 kV in size is applied to the specimen, a pressure wave is generated inside the sample. The pressure wave passes through the sample and is converted into an electrical signal in a piezoelectric element connected to the lower electrode. The pressure wave is amplified 100 times by an amplifier and measured in units of voltage in the oscilloscope 60. These results were transmitted back to the computer 70 through the general purpose interface board (GPIB), and the obtained values were corrected in units of current density. All measurements measured the distribution of charge in the de-energized state after short circuit.

[Examples 1 to 5]

Acrylic acid was grafted to the low density polyethylene in the composition of Table 1 below.

Low density polyethylene was used as the low density polyethylene used for the main insulation of power cables, and acrylic acid was selected from reagent grades produced by Junsei Chemical Co. of Japan, and dicumyl peroxide was used as the organic peroxide. .

The composition ratio of Table 1 shows the weight ratio of each component when the weight of polyethylene which is a base resin is 100.

The composition having the composition ratio of Table 1 was first put in a Henschel mixer, and the monomer and the initiator were mixed in the resin for 20 minutes at a rotation speed of 2000 rpm at 70 ° C. The mixture was reacted and extruded with a twin screw extruder (Ikegai, Japan) having an L / D of 30. At this time, the rotational speed of the extruder was 45rpm, the temperature was set to 165 ~ 200 ℃, the residence time of the resin was to be 1 ~ 1.5 minutes.

Elemental Aanalysis was performed to determine the ratio of acrylic acid grafted in the obtained graft copolymer. Elemental analysis measured oxygen present in each composition because the mother resin, polyethylene, was essentially free of oxygen, but the carboxyl groups in the grafted acrylic acid contained oxygen. Therefore, the concentration of oxygen present in the prepared resin composition is proportional to the grafted ratio and the graft rate is calculated through Equation 1. The measurement results are shown in Table 2 below.

From Table 2, it can be seen that the ratio of grafting increases almost linearly as the amount of acrylic acid added increases.

Using the obtained resin, a disk-shaped specimen having a diameter of 10 cm and a thickness of 500 to 800 µm was prepared. Accumulated charge was measured on the prepared specimens using the apparatus of FIG. 1. In addition, the specimen was prepared as described above using only polyethylene and the amount of accumulated charge was measured in the same manner.

For each specimen, the charge distribution after shorting and applying a voltage of 40 kV at room temperature was shown in FIG. 2 and the accumulated charges on the negative electrode side are shown in Table 3 below. In FIG. 2, the electrode on the left side is the cathode, the electrode on the right side is the anode, and the sign of the measured charge is defined as a positive sign when it appears above the horizontal line and a negative sign when it appears below. In addition, the position of an electrode is shown by the vertical line in FIG.

2 and Table 3, the amount of charge measured for the specimens of the Example was initially increased compared to polyethylene, but was measured to decrease with the increase of the graft rate. In Example 5, it can be seen that the sign of the charge amount is reversed. The sign of the charge amount shows a positive value when the polarity of the voltage applied and the polarity of the charge accumulated in the sample are different, and a negative value in the same case.

Simultaneously with the change of the charge amount sign measured in the example, it can be seen that the order of magnitude decreases by one degree as compared with the grafted polyethylene when the content of the grafted monomer increases. Looking at the amount of positive charge, it increases until Example 2 and then decreases as the graft content increases. Therefore, polyethylene grafted acrylic acid according to the present invention reduced the amount of charge accumulated compared to the base resin when the graft ratio of the monomer is more than about 0.04 wt%, the polarity was also changed at the graft rate of about 0.15 wt%.

In the present invention, the charge distribution was measured for five composition ratios by selecting acrylic acid as the monomer, but may be applied to other composition ranges. In the resin composition grafted acrylic acid, the polyethylene, which is the basic resin, showed heterogeneous charges, and it was determined that the amount of charge accumulated also changed from heterogeneous charges to homogeneous charges as the graft ratio was increased. Therefore, the resin composition of this invention can be used by grafting acrylic acid according to the conditions which require a low accumulated charge amount.

As described above, the polyethylene resin grafted with acrylic acid according to the present invention can control the polarity of the accumulated charge and the magnitude of the charge amount, which can be useful for improving the performance of the insulator of the power cable.

Claims (2)

A resin for charge distribution control, characterized in that acrylic acid as a monomer is grafted onto the main chain of a low density polyethylene resin. A power cable comprising a charge distribution control resin grafted with acrylic acid as a monomer on a main chain of low density polyethylene resin.

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