JPH0440379B2 - - Google Patents

Description

[Detailed description of the invention]

[Technical Field of the Invention] The present invention relates to a highly foamable polyethylene composition, which has excellent electrical properties and shape stability after extrusion molding, does not cause conductor discoloration, and is suitable as an insulator for coaxial cables. It relates to a polyethylene composition with a foaming rate of 65% or more. [Technical background of the invention and its problems] Conventionally, coaxial cables coated with foamed plastic as an insulator have been used in television public viewing systems. In order to minimize the volume and extend the cable connection distance, higher foaming ratios are required. In order to meet these demands, various studies have been conducted, and as a result, coaxial cables with a foaming rate of 70% or more have been manufactured using the so-called gas foaming method, which uses fluorine-based solvents and gases as foaming agents. has been done. However, this method not only increases material costs due to the use of expensive fluorine-based compounds, but also requires mixing these blowing agents with the base plastic at high temperature and pressure, which requires a machine other than an ordinary extruder. The problem was that additional equipment was required for this purpose. On the other hand, chemical foaming using a chemical foaming agent and using an ordinary extruder has the advantage of not requiring special incidental equipment and reducing production costs, but The solidification phenomenon of plastic due to the heat of vaporization that is released during evaporation of dissolved gas cannot be expected, and it is difficult to incorporate a large amount of gas (more than 6 parts by weight) into plastic, making it difficult to manufacture products with a high foaming rate. It was considered difficult to do so. Furthermore, a method of manufacturing highly foamed cables by blending a large amount of chemical foaming agents has been considered, but in this case, the decomposition residue of the foaming agents causes deterioration of the cable's electrical properties such as an increase in tanδ, and the discoloration of the conductors. There was a problem that I could also see it. [Purpose of the Invention] The present invention was made to solve these problems, and it is possible to manufacture a cable insulator with excellent electrical properties and a high foaming rate (65% or more) by feeding it into an ordinary extruder. The purpose of the present invention is to provide a highly foamable polyethylene composition that can be used. [Summary of the invention] Low density polyethylene (hereinafter abbreviated as LDPE)
Or LDPE and high density polyethylene (HDPE)
It is a mixture of azodicarbonamide (hereinafter referred to as ADCA) and p, p' based on 100 parts by weight of base polyethylene having a melt index (hereinafter referred to as MI) of 1 to 7. -oxybis(benzenesulfonyl hydrazide) (hereinafter abbreviated as OBSH) in a weight ratio of 1:1 to
1.0 to 5.0 parts by weight of a blowing agent mixed in a ratio of 4:1 and 10 to 100 parts by weight of azodicarbonamide
The present invention relates to a highly foamable polyethylene composition characterized in that it contains a foaming aid that promotes the decomposition of azodicarbonamide in an amount of % by weight. Here, in the composition of the present invention, the MI value of the base polyethylene was limited to a range of 1 to 7 based on the following experimental results. That is, 1 part ADCA per 100 parts of LDPE with various MI values from 0.2 to 20;
A composition consisting of 0.5 parts of OBSH and 0.5 parts of zinc stearate as a foaming aid is extruded and coated onto an annealed copper wire of 1.8 mm in diameter by a known method and allowed to foam naturally, resulting in a foamed insulated wire core. When the foaming rate was measured and the cross-sectional shape was observed, the results shown in Table 1 below were obtained.

[Table] As is clear from the test results, if the MI of the base polyethylene is too small, the foaming rate cannot be increased, and on the other hand, if the MI is too large, the cross-sectional shape cannot be maintained. Therefore, the MI of the base polyethylene must be in the range of 1 to 7, and more preferably in the range of 2 to 4. In the present invention, ADCA is used as a chemical blowing agent.
and OBSH are used in combination, and the total amount of these blowing agents is in the range of 1.0 to 5.0 parts based on 100 parts of the base polyethylene. The reason why the amount of the blowing agent is limited to this range is as follows. In other words, in order to obtain a cable with a high foaming rate, it is necessary to increase the amount of foaming agent blended.
As the amount of foaming agent added increases, the electrical properties of the cable, such as the dielectric loss tangent (tan δ), tend to decrease. Therefore, when the foaming rate and tan δ were measured by varying the amount of foaming agent mixed, the graph shown in the drawing was obtained. From this graph, we determined the amount of blowing agent that would achieve a high foaming rate and have good electrical properties (tanδ), and found that it was 1.0 parts per 100 parts of base polyethylene.
An optimal range of ~5.0 parts was obtained. In addition, in the present invention, ADCA is used as a blowing agent.
Use a mixture of OBSH and the mixing ratio of 1:1.
The reason for setting the ratio to 4:1 is as follows. In other words, both ADCA and OBSH have a decomposition temperature,
It is widely used as a foaming agent for polyethylene because its gas generation amount, decomposition rate distribution, etc. match well with the processing conditions of polyethylene, and it is easy to handle and inexpensive. When manufacturing cables, there is a problem that when using ADCA alone, the conductor (copper) corrodes (blackens) at an accelerated rate due to the action of a large amount of ammonia contained in the decomposed gas, so OBSH is not used alone. When used in , there was a problem that the cross-sectional shape of the cable became flattened, making it impossible to obtain a round finished shape. Therefore, a mixed blowing agent of ADCA and OBSH is required to neutralize ammonia in the ADCA decomposition gas with an acidic compound that is a decomposition intermediate product of OBSH, and to obtain a cable with a perfectly circular cross section. Base polyethylene with MI of 1.0 and density of 0.92
ADCA and OBSH were mixed in various proportions per 100 parts and extruded onto an annealed copper wire with a diameter of 1.8 mm. The difference in the outer diameter of the insulator and the foaming rate of the obtained cable (insulator outer diameter of 73 mm) were measured. The results shown in Table 2 were obtained.

[Embodiments of the invention]

Next, examples of the present invention will be described. Examples 1 and 2 A polyethylene composition having the composition shown in Table 3 was
The mixture was supplied to a φ extruder, extrusion coated onto annealed copper wire having a diameter of 1.8 mm, and allowed to foam naturally to obtain a foamed insulated wire core. The properties of the obtained insulated wire core are shown in the lower column of the same table.

〔Effect of the invention〕

As is clear from the above description, the composition of the present invention can be extruded and coated onto a conductor using an ordinary extruder to obtain a foamed coaxial cable with a high foaming rate and excellent electrical properties. In addition, the cable thus obtained has a perfectly circular cross-sectional shape without any discoloration of the internal conductor, and has a good finished appearance.

[Brief explanation of drawings]

The drawing is a graph showing the relationship between the experimentally determined blending amount of the foaming agent and the foaming rate and tan δ of the resulting cable.

Claims (1)

[Scope of Claims] 1. A base polyethylene that is low density polyethylene or a mixture of low density polyethylene and high density polyethylene, and has a melt index of 1 to 7.
With respect to 100 parts by weight, azodicarbonamide and p,
1.0 to 5.0 parts by weight of a blowing agent made by mixing p'-oxybis(benzenesulfonyl hydrazide) in a weight ratio of 1:1 to 4:1 and 10 to 100 parts by weight of azodicarbonamide. % of azodicarbonamide. 2. The highly foamable polyethylene composition according to claim 1, wherein the foaming aid is zinc stearate.