JPS58157838A - Polyethylene composition for electrical insulation - Google Patents

Abstract

PURPOSE:The titled composition having improved crosslinking properties, high- speed molding characteristics, electrical tree resistance after crosslinking, electrical strength properties. etc., comprising a low-density polyethylene consisting essentially of a specific high-pressure polyethylene and a crosslinking agent. CONSTITUTION:100pts.wt. low-density polyethylene containing >=70wt% high- pressure polyethylene having 0.924-0.928g/cc, preferably 0.925-0.927g/cc density, 1-5g/10min, preferably 2-4g/10min melt index, 2-5, preferably 3-4 ratio of weight-average molecular weight/number-average molecular weight, 1.25-1.40 die swell ratio, and >=0.2, preferably 0.2-0.5 terminal vinyl group based on 1,000 carbons is blended with 0.5-5pts.wt. crosslinking agent (e.g., dicumyl peroxide, etc.), to give the desired composition.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a polyethylene composition for electrical insulation, which has excellent properties such as electrical strength, electrical resistance, and voltage resistance, and has improved crosslinkability and high-speed moldability.

Specifically, the present invention provides a polyethylene composition for electrical insulation comprising low-density polyethylene and a crosslinking agent, in which the low-density polyethylene (1) has a density of 0.924 to 0.928 f/i, (
2) Melt index is 1 to 5 F/10 minutes, (
3) Weight average molecular weight/number average molecular weight (Mw/un) is 2
~5, (4) Dice sea urchin Tsuta ratio CD, S, R) is 1.25
~1.40, and (5) the number of terminal vinyl groups is 1
Custom-made, high-speed molded columnar tube characterized by using low-density polyethylene whose main component is high-pressure polyethylene with 90.2 or more carbon atoms per 000 carbon, improved electrical properties after crosslinking, especially withstand voltage, electrical resistance The present invention relates to a polyethylene composition for electrical insulation that can provide molded products with excellent tree properties, heat resistance, and appearance.

Conventionally, various glass materials such as Fi have been used as electrical insulating materials. In particular, polyolefin polymers have various special features such as excellent electrical properties, mechanical properties, and chemical stability, but low-density polyethylene has low dielectric loss and can be crosslinked to improve its heat resistance. Widely used as an electrical insulating material for power cables and wires. In general, the properties required for insulating materials for power cables include electrical properties such as voltage resistance, electrical tree resistance, and heat resistance, mechanical strength, durability, high-speed formability, appearance, and frame II Note. It's about being excellent.

In recent years, production! In order to improve I:, speeding up of the production line is required, and there is a demand for the development of an excellent resin that can be cast at high speed without compromising the above-mentioned custom orders.

However, the currently used low-density polyethylene for power cables has the disadvantage that during high-speed molding, flow disturbances called melt air occur and the surrounding surface becomes rough.

The simplest way to improve the above-mentioned high-speed moldability is to increase the melt flow index (hereinafter referred to as Ml), that is, to decrease the molecular weight.
If the MIt ratio is increased, the melt viscosity will be lower.When crosslinking with a crosslinked pipe after forming a wide cable, the molten resin will drip and become eccentric.
Alternatively, troubles may occur if the coated surface comes into contact with the crosslinked pipe and becomes scratched.

On the other hand, one of the important special requirements for resins for power cables is that they have excellent crosslinking properties. This rack II
Factors that contribute to the properties are generally considered to be terminal vinyl groups in intramolecular double bonds, low molecular weight components, short chain branches, long chain branches, etc. Among these, long chain branching is a factor that inhibits crosslinking and reduces the gel fraction. Short chain branching is very complicated in the case of high pressure methods, and its effect is not limited. For low molecular weight components, attempts have been made to improve crosslinking properties by narrowing the molecular weight distribution and reducing f1. For example, in JP-A No. 57-5737, a chemical cross-linking agent is contained, the density is 0.925 f/a1 or more, the crystal thickness is 901 or more, the delivered sardine is 8 or less, and the MFR is 0.
3-4 f/10 min crosslinkable polyethylene has been proposed,
Attempts have been made to improve crosslinking by narrowing the molecular weight distribution and reducing the number of low molecular weight components, but the effect is very small, and it is difficult to obtain products with high-speed moldability and excellent appearance. .

The present inventors have continued to study intensively to improve the shortcomings of conventional electrical insulation resins, particularly resins for electrical cables, and found that the factor that contributes most significantly to the crosslinking properties of high-pressure polyethylene is the unsaturated group within the polyethylene molecule. In particular, we found that the effect of the terminal vinyl group was significant, and as a result of further investigation,
The present invention has been achieved by successfully developing a polyethylene composition that significantly improves all the requirements for the resin for electrical cables, and also exhibits remarkable high-speed moldability and excellent appearance.

That is, the present invention (1) Density is 0.924 to 0.928 f/cc.

(2) Melt index is 1 to 5 F/10 minutes, (3
) Weight average molecular weight/number average molecular weight (Mw/un) is 2~
5. (4) Dice swell ratio CD, S, R) is 1.25 ~
1.40, and (5) the number of terminal vinyl groups is 1000
The present invention provides an electrically insulating polyethylene composition consisting of low density polyethylene whose main component is high-pressure polyethylene containing 0.2 or more carbon atoms and a crosslinking agent. By using this method, the problems of the conventional method can be solved: (1) high-speed formability, (2) voltage resistance, (3) electrical resistance, (4)
Heat resistance, (5) prevention of surface roughness, (6) custom crosslinking,
(7) Volume resistivity (0) and (8) Heat deformation rate, etc., can all be significantly improved. This product also has the advantage that it does not require complicated processes and can be manufactured at low cost.

In the present invention, as the low density polyethylene, the above-described Toohiro (1) density is 0.924 to 0.928 f/cc.

(2) Melt index is 1-5F/10 minutes, (3)
Weight average molecular weight/number average molecular weight (Mw/un) is 2
~5, (4) Dice swell ratio CD, S, R) is 1.25
~1.40, and (5) the number of terminal vinyl groups is 1000
By selecting low-density polyethylene whose main component is high-pressure polyethylene containing 0.2 or more carbon atoms per carbon, various properties of the material can be improved as an electrical insulating material, especially an insulating material for electrical cables.

That is, the present invention provides (1) density t O,924~0
．． 928 t/cc, preferably 0.925-0.9
By setting the temperature to 27f/cc, the heat resistance of Shimika cable at the normal operating temperature is significantly increased from around 90°C to around 100°C, reducing the heat deformation rate and making it possible to increase the total electric power consumption. The above density is (1924f
/cC, uneven flow occurs during molding,
When it exceeds 0.928, surface roughness called shark skin occurs on the coated surface, resulting in poor appearance in any case.

(2) Ml is 1-5 f/10 min, preferably 2-4 f
If the extrusion time is less than 1 F/10 minutes, heat will be generated during extrusion, resulting in scorching, uneven flow, or shark skin, resulting in poor appearance. Cracks occur and uneven thickness occurs.

(3) Molecular weight distribution CMw/un determined by GPC
) is 2 to 5, preferably 3 to 4. If the molecular weight distribution is less than 2, shark skin will occur, and if it exceeds 5 t, uneven flow will occur, and in this case, only a product with poor appearance will be obtained.

(4) Dice swell ratio CD, S, R) is 1.25 to L4
If it exceeds 1.40, uneven flow will occur, and if it exceeds 1.2
If it is less than 5, shark skin will occur and the appearance will be poor.

(5) By setting the number of terminal vinyl groups to 0.2 or more per 100G carbon, a high crosslinking rate of 75% or more can be achieved. , has significantly improved heat resistance above the melting point. When the number of terminal vinyl groups is less than 0.2, the branching and crosslinking rate decreases, and deformation at high temperatures is large, resulting in poor heat resistance. Usually, for example, 0.2
~0.5 pieces can sufficiently achieve the desired effect.

Such high-pressure polyethylene, for example, the molecular weight ratio Mw/
To experimentally obtain low density polyethylene with small Mn,
It can be obtained by separating commercially available low density polyethylene into a plurality of components from low molecular weight components to high molecular weight components, and removing both or either of the low molecular weight components and the high molecular weight components.

In addition, from an industrial perspective, molecular weight ratio Mω/M? Since ethylene polymerization is carried out while keeping these conditions as constant as possible, and by appropriately selecting all polymerization conditions such as polymerization pressure, polymerization temperature, and modifier, it is possible to obtain a high-pressure polymer with a desired molecular weight, molecular weight distribution, etc. Polyethylene is used. For example, by polymerizing with a specific modifier at a low polymerization temperature and further homogenizing, the above (1) to (5)
A specific range of polyethylene can be obtained.

In order to improve high-speed moldability and cross-linking customization, the present invention is an extruder that uses low-density polyethylene containing high-pressure polyethylene as a product component, which has a specific range of materials as described in (1) to (5) above. When extrusion molding is performed, scorch (early crosslinking) in the extruder is completely prevented, allowing for good extrusion processing. The above (1) in low density polyethylene
The amount of high-pressure polyethylene having physical properties within the specific ranges of (5) to (5) is preferably 70% by weight or more.

Examples of low-density polyethylene that can be mixed with such high-pressure polyethylene as necessary include other types of ethylene homopolymers or propylene mainly composed of ethylene,
Butene-1, hexene-1,4-methyl-butene-1
Examples include copolymers with σ-olefins such as, ethylene-vinyl acetate copolymers, etc., and polyethylene obtained by any of high-pressure, medium-pressure, and low-pressure methods may be used.

In addition, the crosslinking agent used in the present invention is a commonly used organic peroxide such as dicumyl peroxide, tert-buterdicumyl peroxide, 2,5-dimethyl-2
, 5-di(tertiary-glutylperoxy)hexano,
2,5-dimethyl-2,5-di(tert-butylperoxy)hexyl-3 etc., usually 0.5 to 5 parts by weight, preferably 1 to 3 parts by weight, per 100 parts by weight of low density polyethylene. The weight part is sourced. Further, if necessary, conventional additives such as voltage stabilizers, copper damage inhibitors, carbon black, etc. may be added in addition to commonly used anti-aging agents.

The method of coating and crosslinking the composition of the present invention on a cable conductor under the usual conditions may be any conventional method such as steam heating or dielectric heating, and is not particularly limited.

As mentioned above, by using the composition of the present invention,
Compared to conventional compositions, it has significantly improved electrical properties such as electrical resistance, electrical tree resistance, and heat resistance, and can be manufactured at low cost without complicated processes.

The present invention will be specifically explained below based on all Examples and Comparative Examples of the present invention, but the present invention is not limited thereto unless it departs from the gist thereof.

Examples 1 to 4 and Comparative Examples 1 to 5 High-pressure polyethylene having the desired properties as shown in Table 1 was produced by changing all polymerization conditions such as modifier, temperature, and pressure using a high-pressure method. The physical properties, electrical properties, moldability, and appearance of the resin were evaluated. The results are shown in Table 1.

As shown in Table 1, Examples 1 to 4 had good electrical properties, gel fraction, high-speed moldability, and good appearance.

In addition, in Comparative Example 1, when the density, M life/us, D, and S-Rt were set outside the range of the present invention, the tree generation voltage, thermal deformation rate, and specific volume resistivity were poor, and the extrusion amount of the resin was poor <2. 3K
I/HR caused uneven flow.

In Comparative Examples 2 and 3, Mlt was set outside the range of the present invention, and in Comparative Example 2, the extrusion amount was L5Kf/HR, resulting in small flow irregularities, and in Comparative Example 3, drawdown occurred and the resin drooped.

In Comparative Example 4, when the density was set outside the range of the present invention, the extrusion rate was as low as 3.5 h/HB, and shark skin was produced.

In Comparative Example 5, when the vinyl group at the end of the line was set outside the range of the present invention, a product with good moldability and appearance was obtained, but the gel fraction was low.

Example 5 MI=3 in the high-level part of the low-density polyethylene 100 of Example 1
, 10 parts of ethylene-vinyl acetate copolymer having a density of 0.933 and a VA content of 10 cm were added, and mixed with the extruder. The composition was tested in the same manner as in Example 1 and showed good properties. The results are shown in Table 1.

Comparative Example 6 Commercially available high density polyethylene density 0.925, MI=0.
2 30 parts by weight of polyethylene and density 0.925~MI=
70 parts by weight of polyethylene of No. 10 was pre-coated with an extruder,
As a result of testing with a composition outside the range of the present invention having MI = 3, Mw/Hori = 5.5, DSR = 1.43, and a terminal vinyl group of 0.29, flow irregularity occurred at an extrusion rate of 3.8 KI/HR. occurred.

Test method〉 Die swell ratio... Diameter CD of the strand extruded from the melt indexer) and diameter CD0 of the orifice
) was calculated using the following formula.

DSR=Example.

Vinyl terminal... 0.6X thick sheet. Using the infrared absorption spectrum, the absorbance at the wavelength ii and oaμ was determined from the following formula: o 1 ': J/1000C = °lOgTx ■ Here , K=0
．． 116, d: Density variation of PE, l: Sheet thickness, l0
2I: Custom-made absorption and base absorbance electrical tree... Using a needle with a radius of curvature of 3 μm, distance between LA electrodes 3X, initial applied voltage 5kV, 10 minutes test 'R:, 1 & τ step-up single needle test, 10 The voltage at which trees occurred in five of the samples was determined.

Heating deformation rate... 6X thickness seam with diameter 1ON-)
't Pressurized at a load of 2.63 Kt in an oil bath at 100°C, and the deformation rate after 30 minutes was determined.

Volume specific resistivity ψ)...0.3% thickness receipt charge,
pt was determined at an electric field strength of 10 kv/.

Gel fraction: 100wt part of polyadelene, Zwt part of DCP digyl peroxide, 4.4'
-thiobis(2-t-butyl-5-methylphene/-1
) Add all 0.2wt part and form IX sheet lA16
Crosslinking was carried out at 0°C for 30 minutes, and then pulverized into 20 mesh passes, extracted with xylene at 120°C for 10 hours, and the residual ratio was determined.

High-speed moldability: Using a blow molding machine and using a die with an inner diameter of 9XΦ and an outer diameter of 10%Φ, the extrusion volume was determined when the surface became rough as the extrusion volume was increased. Resin temperature 130℃.

Reference Example High-pressure polyethylene with physical properties of A, B, C, and D4111 was produced by changing the polymerization conditions such as modifier, temperature, and pressure using the high-pressure method, and the effects of crosslinking were examined. Table 2 shows the results. Indicated.

Table 2 Ml fAO min 3.0 3.0
2.8 1.7 density f/ed O,92
30,9250,9280,930Mω/Mt*
-4,23,43,02,8 low molecular weight component %
3.8 3.1 2.1 0.9 terminal vinyl group Machi o”G o, ai O,31
0,150,03 As shown in Table 2 above, A-+B
−+C−+D and Mt2I/ which is an index of molecular weight distribution in order
If M'nf is small, that is, the molecular weight distribution is narrowed, the low molecular weight components will obviously decrease, but the gel fraction will be A.

B is the highest, whereas C and D are the lowest. In addition, in A and B having the same terminal vinyl group, khl)/
Mt* has not changed. It is clear from the results that the terminal vinyl group has a large influence on crosslinking properties.

Claims (1)

[Claims] A polyethylene composition for an electric cable comprising low density polyethylene and a crosslinking agent, wherein the low density polyethylene (1) has a density of 0.924 to 0.928 f/CC.
. (2) Melt index is 1 to 5 f710 minutes, (3
) weight average molecular weight/number average molecular weight (Ih1w/M%) is 2 to 5, (4) die swell ratio CD, S, R) is 125
~1.40, and (5) the number of terminal vinyl groups is 1000
A polyethylene composition for electrical insulation, characterized in that it is a low-density polyethylene whose main component is high-pressure polyethylene having a carbon content of 0.2 or more.