JPH0578124B2 - - Google Patents

Description

[Detailed description of the invention]

[Technical Field of the Invention] The present invention relates to a method for producing a highly foamed polyethylene insulated wire having a foaming rate of 80% or more and excellent mechanical strength by a gas foaming method using a low boiling point liquid foaming agent. [Technical background of the invention and its problems] In recent years, in coaxial cables, etc., the so-called high foaming technology has been used to increase the foaming rate of the insulator, thereby reducing the dielectric constant, tanδ, and leakage attenuation. Coaxial cables are being developed. Conventionally, the method for producing coaxial cable cores with high expansion ratios has been to feed low-density polyethylene mixed with a low-boiling point liquid foaming agent such as Freon (trade name) to an extruder under pressure and extrude it onto the conductor. Gas foaming is known, in which the coating is covered, then foamed in air, and then cooled and solidified.
% or more. However, when trying to obtain an insulating layer with a high foaming rate of 80% or more using the gas foaming method, the conventional methods of (a) increasing the amount of foaming agent blended, and (b) increasing the conductor temperature (c) Methods such as increasing the extrusion temperature have the disadvantage that the cells become continuous, making it difficult to achieve a high foaming rate and reducing mechanical strength. Another problem with method (a) was that the manufacturing cost was high. [Object of the Invention] The present inventors have addressed these conventional circumstances by suppressing the continuity of cells during molding of highly foamed insulators, and
After repeated studies to develop a base polymer that can also improve mechanical strength, we found that melt viscosity (MFR) and melt elasticity (swelling ratio), which have traditionally been used as selection indicators for base polymers for foam compositions, were insufficient. However, it was not possible to select a highly foamed insulator with good properties, and it was discovered that the melt tension of the polymer had a large effect on the above properties. In other words, when we continuously measured the capacitance of highly foamed foamed polyethylene insulated wire on an extrusion line, we observed a phenomenon in which the capacitance suddenly increased due to water entering the continuous foam. However, increasing the melt tension of the polymer improves the strength of the bubble walls, prevents the bubbles from becoming continuous, and eliminates this phenomenon. As a result of further research, they discovered that when medium- and high-density polyethylene are blended into low-density polyethylene at a predetermined ratio, the melt tension at the extrusion temperature is higher than that of each of the original polymers. The present invention was made based on such knowledge, and it suppresses the continuity of bubbles in the gas foaming method,
The object of the present invention is to provide a method for producing a highly foamed polyethylene insulated wire having a high foaming rate of 80% or more and excellent electrical and mechanical properties. [Summary of the Invention] That is, the method for manufacturing a highly foamed polyethylene insulated wire of the present invention comprises a method for manufacturing a high-foam polyethylene insulated wire using 50 to 80% by weight of low density polyethylene.
It is characterized by extruding and foaming a composition consisting of polyethylene with a melt viscosity (MFR) of 3 or more and a low boiling point liquid foaming agent, which is made of polyethylene containing 20 to 50% by weight of medium and high density polyethylene and 20 to 50% by weight of medium and high density polyethylene and mixed with a low boiling point liquid foaming agent. Here, the melt viscosity (MFR) is a value measured according to JIS K 7210 (condition E 190°C, 2.16Kg ⁺ ). In the present invention, the blending ratio of low density polyethylene and medium/high density polyethylene as the base polymer is limited to 50 to 80% by weight of low density polyethylene and 20 to 50% by weight of medium/high density polyethylene.
This is because if the high density polyethylene is less than 20% by weight, its physical properties are nearly as inadequate as those of low density polyethylene alone, and the effect of blending medium/high density polyethylene is hardly visible. % or less because if it exceeds 50% by weight, the poor extrusion processability of medium- and high-density polyethylene becomes apparent. When low-density polyethylene and medium- and high-density polyethylene are blended in the above range, the melt tension increases above the inherent value of the original single polymer, as described above, and this phenomenon is particularly This is noticeable on the low temperature side. Table 1 shows the experimental results showing this phenomenon, and shows the results for low-density polyethylene (manufactured by Ube Industries, Ltd.) of 1 type and medium/high-density polyethylene (manufactured by Mitsui Petrochemicals, Inc.) of 2 types.
This is a summary of the results of measuring the melting properties of seeds and polymers mixed with them. In Table 1, as mentioned above, the melt viscosity (MFR) is based on JIS K 7210, and the swelling ratio is
From the outer diameter ds of the extrudate obtained when measuring the melt viscosity (MFR) according to JIS K 7210 and the orifice inner diameter d ₀ , the following formula is used: Swelling ratio = (ds - d ₀ /d ₀ ) x 100 (%) This is the obtained value. In addition, the melt tension was measured using a melt tension tester with an orifice inner diameter of 2.095 mm and a length of 8 mm.
Strands extruded at an extrusion speed of 20 mm/min.
This is a value measured by pulling at a speed of 10 m/min.

【table】

[Table] Figures 1 and 2 are graphs showing the temperature characteristics of the melt tension shown in Table 1, and it is clear that the melt tension increases due to the blending of low density polyethylene and medium/high density polyethylene. Is recognized. Therefore, in the present invention, the polymer in which low density polyethylene and medium/high density polyethylene are mixed exhibits the phenomenon of increase in melt tension as described above at least at its extrusion temperature. Density polyethylene is
Due to its formulation, it exhibits a phenomenon of increasing melt tension at least at its extrusion temperature and has a high melt tension. In addition, in the present invention, the melt viscosity (MFR) is
The reason for the above limitation is that if the melt viscosity (MFR) is less than 3, foaming will occur explosively, making it impossible to obtain uniform, fine, and good foaming. In the present invention, a known low-boiling liquid blowing agent such as a halogenated hydrocarbon such as liquid Freon or a low-boiling hydrocarbon such as butane, pentane, or hexane is dissolved and mixed in the base polymer as described above, and the outer periphery of the conductor is heated in advance. extruded into foam. The amount of foaming agent mixed and extrusion conditions are selected appropriately taking into account the desired foaming rate, but they should be selected to the extent that problems such as open cells and reduced economic efficiency do not occur. A high foaming rate can be obtained. [Examples of the invention] Next, examples of the invention will be described. Example 60% by weight of low-density polyethylene with a density of 0.919 g/cm ³ and MFR 3.07 shown in Table 1, and a density of 0.944 g/cm ³ ,
40% by weight of medium density polyethylene of MFR 7.13 was mixed in a roll mixer and pelletized. Next, the pellets are mixed with 11 to 12 parts by weight of liquid Freon 114 (trade name) per 100 parts by weight of polyethylene and fed into an extruder, and extruded onto the conductor at a temperature of 135 to 135 parts by weight.
Extrusion foaming at 140° to create a foam insulation layer outer diameter of 8.0~
Manufactured 8.2mm coaxial cable core. The capacitance of the obtained coaxial cable core was measured by wrapping aluminum foil around the outer periphery of the foamed insulating layer, and the compressive strength of the foamed insulating layer was also measured. The capacitance of this coaxial cable core is 49.2 to 50.7 (PF/m),
The bubbles were uniformly formed. Furthermore, when a coaxial cable was manufactured using this coaxial cable core with an aluminum outer conductor and outer sheath, it had a capacitance of 50.2nF/Km and exhibited other good characteristics. The above measurement results are shown in FIG. In addition, those shown as comparative examples in the table have a density of 0.928 g/cm ³ ,
These are the characteristics of a conventional highly foamed coaxial cable using low-density polyethylene with an MFR of 2.97, a swelling ratio of 1.44, and a melt tension of 3.5 g at 170°C.

[Table] Table 2 shows that the capacitance of the coaxial cable of the example is lower than that of the conventional one, and the mechanical strength of the cable is 81.2% with a foaming rate of 81.2% compared to that of the conventional cable with a foaming rate of 74%.
% cable strength. [Effects of the Invention] As is clear from the above examples, the method of the present invention produces a highly foamed polyethylene insulated wire with a foaming rate of 80% or more, a good foaming state, and excellent electrical and mechanical properties. Obtainable.

[Brief explanation of the drawing]

FIGS. 1 and 2 are graphs showing the temperature characteristics of melt tension of low density polyethylene, medium and high density polyethylene, and mixtures thereof.

Claims (1)

[Claims] 1. A composition consisting of 50 to 80% by weight of low density polyethylene and 20 to 50% by weight of medium/high density polyethylene with a melt viscosity (MFR) of 3 or more mixed with a low boiling point liquid blowing agent is extruded onto the outer periphery of the conductor. A method for producing a highly foamed polyethylene insulated wire, which is characterized by foaming.