JP3187490B2 - Polypropylene for electrical insulation and method for producing the same - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a polypropylene for electrical insulation and a method for producing the same.

[0002]

2. Description of the Related Art Polypropylene is used alone or as a blend polymer with low-density crosslinked polyethylene or the like, for example, as an electrical insulating material for high-voltage cables and equipment. Polypropylene is a highly crystalline polymer. If the polypropylene is cooled from a molten state, particularly slowly cooled, the lamella grows radially around the nucleus, and a spherulite of about 100 to 200 μm is generated. Then, when spherulites grow sufficiently from a large number of nuclei, collision occurs between the spherulites to form a spherulite interface.

However, the crystalline polypropylene obtained by the above method cannot be said to have a sufficient impulse breakdown electric field strength, and when this is used at a high voltage as an electrical insulating material,
There are inconveniences such as easy breakdown. Therefore,
There has been a long-awaited need for a higher-performance and more reliable polypropylene for electrical insulation having excellent impulse breakdown electric field strength and a production method thereof.

[0004]

Means for Solving the Problems When the present inventors made impulse fracture of the polypropylene crystal obtained as described above, the impulse fracture passed through the spherulite interface, and the spherulite interface became an electrical weak point. And further studies have revealed that the average spherulite diameter is 20 μm or less,
It has been found that polypropylene preferably having a spherulite of 10 μm or less has excellent impulse breakdown electric field strength.

The present invention has been completed on the basis of such new findings, and relates to a polypropylene for electrical insulation characterized by comprising spherulites having an average spherulite diameter of 20 μm or less, preferably 10 μm or less.

The present invention also relates to a method for producing the above-mentioned polypropylene for electrical insulation, which comprises a step of adding a nucleating agent to polypropylene, melting the polypropylene, and then cooling the polypropylene. In the present invention, the average spherulite diameter of polypropylene is 2
It is 0 μm or less, preferably 10 μm or less, more preferably 5 μm or less, and still more preferably 2 μm or less. The average spherulite diameter can be measured as follows. That is, the polypropylene for electrical insulation of the present invention
The slice can be sliced from about μm to about 50 μm, and this is observed with a polarizing microscope to measure the average spherulite diameter.

As a method for producing polypropylene having the average spherulite diameter, for example, the following method is exemplified.
A method of crystallization by adding a nucleating agent to polypropylene. In the method, a nucleating agent having the following characteristics is preferable. Good compatibility with polypropylene. Here, those having good compatibility with polypropylene are the average of the dispersed particles when the particles are kneaded at the melting point of the polypropylene (170 to 180 ° C.) and the dispersion state of the particles is observed with an optical microscope (× 400) at that temperature. The diameter is usually 1 μm or less, preferably 0.1 μm.
It refers to those having a size of 5 μm or less. Those with nucleation properties. Here, the nucleating property has an effect of reducing the average spherulite of polypropylene to 20 μm or less by performing crystallization under ordinary crystallization conditions after mixing with polypropylene.

Particularly preferred nucleating agents include the following formula (1)

[0009]

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(Wherein, X ₁ and X ₂ each represent a hydrogen atom or an organic group having 10 or less carbon atoms). In the formula (1), examples of the organic group having 10 or less carbon atoms include an alkyl group and an alkoxy group. As the alkyl group, a lower alkyl group having 1 to 4 carbon atoms such as methyl, ethyl, propyl, isopropyl and butyl is preferable. As the alkoxy group, a lower alkoxy group having 1 to 4 carbon atoms such as methoxy, ethoxy, propoxy, isopropoxy and butoxy is preferable. Particularly preferred compounds of the formula (1) include dibenzylidene sorbitol, di (paraethyl) benzylidene sorbitol, di (paramethoxy) benzylidene sorbitol and the like. The compounding amount of the nucleating agent is usually 0.2 to 5 parts by weight, preferably 0.5 to 3 parts by weight based on 100 parts by weight of polypropylene.

In the present invention, the raw material polypropylene has a melt index of 3 to 11, preferably 5 to 7, and a density of 0.905 to 0.915, preferably 0.908 to 0.98.
0.912 and the elongation at break is 350-5
Preferably, it is 50%, especially 450-550%.
The melt index (g / 10 minutes) is based on JIS K
It was measured by the method of 6760K. The elongation at break is measured by ASTND 638.

A method using a nucleating agent is carried out, for example, as follows. First, polypropylene and a nucleating agent are kneaded. The kneading is usually performed using a roll mill or the like. The temperature at that time is usually 170 to 200 ° C, preferably 180 to 190 ° C. The kneaded material is then melted.
Melting is at a temperature higher than the melting point of polypropylene, that is, usually 1
Performed at 70-180 ° C. Melting time is usually 10
It takes about 20 minutes. The melt thus obtained undergoes isothermal crystallization at an appropriate temperature and time. Isothermal crystallization temperature (℃)
Is usually 130 to 140 ° C., preferably 132 to 138
° C, more preferably 134 to 136 ° C.

The isothermal crystallization time (min) is usually 30 to 12
0, preferably 60-90, more preferably 70-80
It is. After isothermal crystallization, it is subjected to a cooling step. The cooling temperature at that time is usually 0 to 50 ° C, preferably 10 to 4 ° C.
0 ° C., more preferably 20-30 ° C., and the cooling rate is usually 10-300 ° C./min, preferably 60-250 ° C./min.
Min, more preferably 120 to 200 ° C./min.

[0014]

The impulse breakdown electric field strength of the polypropylene for electrical insulation of the present invention is about 1.6 times better than that of the conventional crystalline polypropylene.

[0015]

【Example】

Example 1 Isotactic polypropylene having a density of 0.91 g / cm ³ , a melt index of 7.0, and an elongation at break of 500% was used. Benzal sorbitol (Clear-S: manufactured by Nippon Rika Co., Ltd.) was used as a nucleating agent, kneaded with the polypropylene at 180 ° C. using a roll mill, and once melted (180 ° C.) in a compression molding machine. After isothermal crystallization at 130 ° C for 60 minutes, polypropylene was crystallized by rapid cooling with 0 ° C water. The average spherulite diameter of the polypropylene is, for example, about 5 μm when 0.3 phr of Clear S is added.
m. This was compression molded into a 0.3 mm thick sheet.

Experimental Example 1 An impulse breakdown test was performed on the sample obtained above.
That is, in the improved McKeown electrode system shown in FIG.
A negative impulse standard wave of × 40 μsec was applied with a step-up voltage of 5 kV / 3 times, with 70% of the expected breakdown voltage as an initial value. The sample thickness is 0.3 mm.
As for the impulse withstand voltage value, data of 10 samples was collected under one condition, and after the wipe-pull analysis, the impulse withstand voltage value of the sample was defined as the destruction value at a probability of failure of 63.3%. The measurement results of the impulse electric field breakdown strength [ImpBDS (kV / mm)] thus measured are as shown in FIG. 1 and Table 1. In FIG. 1, phr indicates parts by weight of the nucleating agent with respect to 100 parts by weight of polypropylene.

[0017]

[Table 1]

The amounts of addition in the table are parts by weight of the nucleating agent per 100 parts by weight of polypropylene.

The compatibility of benzalsorbitol used in the Examples with polypropylene was determined by kneading the polypropylene with 170 ° C. and then using an optical microscope (× 40) at that temperature.
When observed in 0), the particles were small enough that no dispersed particles were observed.

Comparative Example 1 The polypropylene used in Example 1 was once melted (180 ° C.) in a molding machine, crystallized isothermally at 140 ° C. for 3 hours, and left at 25 ° C. for 2 hours to crystallize the polypropylene. Was. The spherulite diameter of the polypropylene crystal was 25 μm. When the impulse breakdown electric field strength of the crystalline polypropylene was measured in the same manner as in Experimental Example 1, it was 120 k
V / mm.

[Brief description of the drawings]

FIG. 1 is a diagram showing a measurement result of an impulse electric field breakdown strength.

FIG. 2 is an improved McK for performing an impulse breakdown test.
FIG. 3 is a diagram illustrating an eown electrode system.

[Explanation of symbols]

 1 25mmφ steel ball 2 polyethylene body 3 silicon oil 4 epoxy resin 5 sample

──────────────────────────────────────────────────続 き Continuation of the front page (56) References JP-A-61-54108 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) H01B 3/44 C08L 23/10 C08F 110 / 06

Claims (2)

(57) [Claims] 1. An electrical insulating polypropylene comprising a spherulite having an average spherulite diameter of 20 μm or less, which is obtained by adding a nucleating agent to polypropylene and crystallizing the same. 2. A cable using the polypropylene for electrical insulation according to claim 1.